fix(transpiler/dart): re-exporting only some bindings

```
export {Foo, Bar} from ‘./foo’;
==>
export ‘./foo’ show Foo, Bar;
```

tools/transpiler/spec/bar.js

@@ -0,0 +1,3 @@
+export var Bar1 = 'BAR1';
+export var Bar2 = 'BAR2';
+export function Bar3() {}

tools/transpiler/spec/export.js

@@ -1 +1,2 @@
 export * from './foo';
+export {Bar1, Bar2} from './bar';

tools/transpiler/spec/imports_spec.js

@@ -1,4 +1,4 @@
-import {describe, it, expect} from 'test_lib/test_lib';
+import {describe, it, expect, IS_DARTIUM} from 'test_lib/test_lib';
 
 import {Foo, Bar} from './foo';
 // TODO: Does not work, as dart does not support renaming imports
@@ -21,6 +21,18 @@ export function main() {
 
       expect(exportModule.Foo).toBe('FOO');
       expect(exportModule.Bar).toBe('BAR');
+      expect(exportModule.Bar1).toBe('BAR1');
+      expect(exportModule.Bar2).toBe('BAR2');
+
+      // Make sure Bar3 is not re-exported.
+      expect(function() {
+          exportModule.Bar3();
+        }).toThrowError(IS_DARTIUM ?
+          // Dart
+          "No top-level method 'exportModule.Bar3' declared.":
+          // JavaScript
+          'undefined is not a function'
+        );
 
       expect(Type).toBeTruthy();
     });

tools/transpiler/src/outputgeneration/DartParseTreeWriter.js

@@ -25,6 +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';
 
 export class DartParseTreeWriter extends JavaScriptParseTreeWriter {
   constructor(moduleName, outputPath) {
@@ -275,12 +276,34 @@ export class DartParseTreeWriter extends JavaScriptParseTreeWriter {
   // EXPORTS
   visitExportDeclaration(tree) {
     if (tree.declaration.moduleSpecifier) {
-      this.write_('export');
+      if (tree.declaration.specifierSet.type === EXPORT_STAR) {
-      this.writeSpace_();
+        // export * from './foo'
-      this.visitModuleSpecifier(tree.declaration.moduleSpecifier);
+        // ===>
-      this.write_(SEMI_COLON);
+        // 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 {
-      // remove "export"
+      // Just remove the "export" keyword.
+      // export var x = true;
+      // export class Foo {}
+      // ===>
+      // var x = true;
+      // class Foo {}
       this.writeAnnotations_(tree.annotations);
       this.visitAny(tree.declaration);
     }