feat(transiler/dart): re-export imported vars

```
import {Foo} from ‘./foo’;
var localVar = true;
export {Foo, localVar};

===>

import ‘./foo’ show Foo;
export ‘./foo’ show Foo;
var localVar = true;
```

Closes #41

tools/transpiler/spec/baz.js

@@ -0,0 +1 @@
+export var Baz = 'BAZ';

tools/transpiler/spec/imports_spec.js

@@ -9,8 +9,14 @@ import * as exportModule from './export';
 
 import {Type} from 'facade/lang';
 
+import {Baz} from './reexport';
+
 export function main() {
   describe('imports', function() {
+    it('should re-export imported vars', function() {
+      expect(Baz).toBe('BAZ');
+    });
+
     it('should work', function() {
       expect(Foo).toBe('FOO');
       expect(Bar).toBe('BAR');

tools/transpiler/spec/reexport.js

@@ -0,0 +1,10 @@
+import {Baz} from './baz';
+import {Bar1} from './bar';
+
+var localVar = true;
+
+export {Baz, localVar, Bar1};
+
+// Will become:
+// export {Baz} from './baz';
+// export {Bar1} from './bar';

tools/transpiler/src/codegeneration/DartTransformer.js

@@ -8,6 +8,7 @@ import {InstanceOfTransformer} from './InstanceOfTransformer';
 import {MultiVarTransformer} from './MultiVarTransformer';
 import {StrictEqualityTransformer} from './StrictEqualityTransformer';
 import {NamedParamsTransformer} from './NamedParamsTransformer';
+import {ExportTransformer} from './ExportTransformer';
 
 /**
  * Transforms ES6 + annotations to Dart code.
@@ -28,5 +29,6 @@ export class DartTransformer extends MultiTransformer {
     append(InstanceOfTransformer);
     append(StrictEqualityTransformer);
     append(ClassTransformer);
+    append(ExportTransformer);
   }
 }

tools/transpiler/src/codegeneration/ExportTransformer.js

@@ -0,0 +1,108 @@
+import {ParseTreeTransformer} from './ParseTreeTransformer';
+import {Map} from 'traceur/src/runtime/polyfills/Map';
+import {
+  ExportDeclaration,
+  ExportSpecifierSet,
+  NamedExport,
+  EmptyStatement
+} from 'traceur/src/syntax/trees/ParseTrees';
+import {
+  IMPORT_DECLARATION,
+  NAMED_EXPORT
+} from 'traceur/src/syntax/trees/ParseTreeType';
+
+
+// Return the index of the first item that is not of IMPORT_DECLARATION type.
+function getIndexOfFirstNonImportStatement(items) {
+  var index = 0;
+
+  while (index < items.length && items[index].type === IMPORT_DECLARATION) {
+    index++;
+  }
+
+  return index;
+}
+
+
+/**
+ * Transforms re-exports:
+ * ```
+ * import {Foo} from './foo';
+ * import {Bar} from './bar';
+ * var localVar = true;
+ *
+ * export {Foo, Bar, localVar}
+ *
+ * ===>
+ *
+ * import {Foo} from './foo';
+ * import {Bar} from './bar';
+ * var localVar = true;
+ *
+ * export {Foo} from './foo';
+ * export {Bar} from './bar';
+ * ```
+ *
+ * In Dart, all variables defined in the root context of a module that don't start with
+ * an underscore are exported. Thus we just drop the "export" keyword for the locally defined vars.
+ * Variables imported from other modules need to be exported with `export './foo' show Foo`.
+ * This transformer drops the local exports and add the information about module path for the
+ * imported variables.
+ */
+export class ExportTransformer extends ParseTreeTransformer {
+  constructor(idGenerator, reporter) {
+    this.reporter_ = reporter;
+    this.importedVars_ = null;
+    this.collectedExports_ = null;
+  }
+
+  transformModule(tree) {
+    // New context for each file (module).
+    this.importedVars_ = new Map();
+    this.collectedExports_ = [];
+
+    tree = super.transformModule(tree);
+
+    // In Dart, imports and exports have to be at the top, before any other statement.
+    // Insert the collected exports before the first non-import statement (after all imports).
+    var items = tree.scriptItemList;
+    var index = getIndexOfFirstNonImportStatement(items);
+    tree.scriptItemList = items.slice(0, index).concat(this.collectedExports_, items.slice(index));
+
+    return tree;
+  }
+
+  // For each imported variable, store the module path where it comes from.
+  // For instance, `import {Foo} from './foo'` will map 'Foo' -> './foo'.
+  // TODO(vojta): deal with `import * as m from './foo'`.
+  transformImportDeclaration(tree) {
+    tree.importClause.specifiers.forEach((specifier) => {
+      this.importedVars_.set(specifier.binding.binding.identifierToken.value, tree.moduleSpecifier);
+    });
+
+    return tree;
+  }
+
+  transformExportDeclaration(tree) {
+    if (tree.declaration.type === NAMED_EXPORT && tree.declaration.moduleSpecifier === null) {
+      // export {...}
+      tree.declaration.specifierSet.specifiers.forEach((specifier) => {
+        // Filter out local variables, keep only imported ones.
+        if (!this.importedVars_.has(specifier.lhs.value)) {
+          return;
+        }
+
+        // For each specifier, create a new ExportDeclaration and attach the module path (collected
+        // in `transformImportDeclaration`) to it.
+        this.collectedExports_.push(new ExportDeclaration(tree.location,
+            new NamedExport(tree.declaration.location, this.importedVars_.get(specifier.lhs.value),
+              new ExportSpecifierSet(tree.declaration.specifierSet.location, [specifier])),
+            tree.annotations));
+      });
+
+      return new EmptyStatement(tree.location);
+    }
+
+    return tree;
+  }
+}

tools/transpiler/src/outputgeneration/DartParseTreeWriter.js

@@ -25,7 +25,7 @@ import {
 import {ParseTreeWriter as JavaScriptParseTreeWriter, ObjectLiteralExpression} from 'traceur/src/outputgeneration/ParseTreeWriter';
 import {ImportedBinding, BindingIdentifier} from 'traceur/src/syntax/trees/ParseTrees';
 import {IdentifierToken} from 'traceur/src/syntax/IdentifierToken';
-import {EXPORT_STAR} from 'traceur/src/syntax/trees/ParseTreeType';
+import {EXPORT_STAR, NAMED_EXPORT} from 'traceur/src/syntax/trees/ParseTreeType';
 
 export class DartParseTreeWriter extends JavaScriptParseTreeWriter {
   constructor(moduleName, outputPath) {
@@ -34,6 +34,8 @@ export class DartParseTreeWriter extends JavaScriptParseTreeWriter {
     this.annotationContextCounter = 0;
   }
 
+  visitEmptyStatement() {}
+
   // CLASS FIELDS
   visitPropertyVariableDeclaration(tree) {
     if (tree.isStatic) {
@@ -275,35 +277,43 @@ export class DartParseTreeWriter extends JavaScriptParseTreeWriter {
 
   // EXPORTS
   visitExportDeclaration(tree) {
-    if (tree.declaration.moduleSpecifier) {
-      if (tree.declaration.specifierSet.type === EXPORT_STAR) {
-        // export * from './foo'
-        // ===>
-        // export './foo';
-        this.write_('export');
-        this.writeSpace_();
-        this.visitModuleSpecifier(tree.declaration.moduleSpecifier);
-        this.write_(SEMI_COLON);
+    if (tree.declaration.type === NAMED_EXPORT) {
+      // export {...}
+      // export {...} from './foo'
+      // export * from './foo'
+
+      if (tree.declaration.moduleSpecifier) {
+        if (tree.declaration.specifierSet.type === EXPORT_STAR) {
+          // export * from './foo'
+          // ===>
+          // export './foo';
+          this.write_('export');
+          this.writeSpace_();
+          this.visitModuleSpecifier(tree.declaration.moduleSpecifier);
+          this.write_(SEMI_COLON);
+        } else {
+          // export {Foo, Bar} from './foo'
+          // ===>
+          // export './foo' show Foo, Bar;
+          this.write_('export');
+          this.writeSpace_();
+          this.visitModuleSpecifier(tree.declaration.moduleSpecifier);
+          this.writeSpace_();
+          this.write_('show');
+          this.writeSpace_();
+          this.writeList_(tree.declaration.specifierSet.specifiers, COMMA, false);
+          this.write_(SEMI_COLON);
+        }
       } else {
-        // export {Foo, Bar} from './foo'
-        // ===>
-        // export './foo' show Foo, Bar;
-        this.write_('export');
-        this.writeSpace_();
-        this.visitModuleSpecifier(tree.declaration.moduleSpecifier);
-        this.writeSpace_();
-        this.write_('show');
-        this.writeSpace_();
-        this.writeList_(tree.declaration.specifierSet.specifiers, COMMA, false);
-        this.write_(SEMI_COLON);
+        // export {...}
+        // This case is handled in `ExportTransformer`.
+        throw new Error('Should never happen!');
       }
     } else {
-      // Just remove the "export" keyword.
-      // export var x = true;
+      // export var x = true
       // export class Foo {}
-      // ===>
-      // var x = true;
-      // class Foo {}
+      // export function bar() {}
+      // Just remove "export" keyword.
       this.writeAnnotations_(tree.annotations);
       this.visitAny(tree.declaration);
     }