refactor(benchpress): make tests for error cases also work in Dart

Also introduces `PromiseWrapper.catchError`.
Could not use `PromiseWrapper.catch` as a name as Dart would not allow
this method name.

modules/angular2/src/facade/async.dart

@@ -15,6 +15,12 @@ class PromiseWrapper {
     return promise.then(success, onError: onError);
   }
 
+  // Note: We can't rename this method to `catch`, as this is not a valid
+  // method name in Dart.
+  static Future catchError(Future promise, Function onError) {
+    return promise.catchError(onError);
+  }
+
   static _Completer completer() => new _Completer(new Completer());
 
   static void setTimeout(fn(), int millis) {

modules/angular2/src/facade/async.es6

@@ -12,6 +12,12 @@ export class PromiseWrapper {
     return Promise.reject(obj);
   }
 
+  // Note: We can't rename this method into `catch`, as this is not a valid
+  // method name in Dart.
+  static catchError(promise:Promise, onError:Function):Promise {
+    return promise.catch(onError);
+  }
+
   static all(promises:List):Promise {
     if (promises.length == 0) return Promise.resolve([]);
     return Promise.all(promises);

modules/benchpress/test/web_driver_extension_spec.js

@@ -1,7 +1,8 @@
 import {ddescribe, describe, it, iit, xit, expect, beforeEach, afterEach} from 'angular2/test_lib';
 
 import { StringMap, ListWrapper } from 'angular2/src/facade/collection';
-import { isPresent, StringWrapper, isJsObject } from 'angular2/src/facade/lang';
+import { isPresent, StringWrapper } from 'angular2/src/facade/lang';
+import { PromiseWrapper } from 'angular2/src/facade/async';
 
 import { WebDriverExtension, bind, Injector, Options } from 'benchpress/benchpress';
 
@@ -23,16 +24,15 @@ export function main() {
       });
     });
 
-    // TODO(tbosch): In Dart, somehow we don't provide the error
-    // correctly in the promise result...
-    if (isJsObject({})) {
     it('should throw if there is no match', (done) => {
-        createExtension(['m1'], {'browser': 'm2'}).then(null, (err) => {
+      PromiseWrapper.catchError(
+        createExtension(['m1'], {'browser': 'm2'}),
+        (err) => {
           expect(isPresent(err)).toBe(true);
           done();
-        });
-      });
         }
+      );
+    });
 
   });
 }

modules/benchpress/test/webdriver/chrome_driver_extension_spec.js

@@ -2,7 +2,7 @@ import {describe, it, iit, xit, expect, beforeEach, afterEach} from 'angular2/te
 
 import { ListWrapper } from 'angular2/src/facade/collection';
 import { PromiseWrapper } from 'angular2/src/facade/async';
-import { Json, isBlank, isJsObject } from 'angular2/src/facade/lang';
+import { Json, isBlank } from 'angular2/src/facade/lang';
 
 import {
   WebDriverExtension, ChromeDriverExtension,
@@ -164,20 +164,16 @@ export function main() {
         });
       });
 
-      // TODO(tbosch): In Dart, somehow we don't provide the error
-      // correctly in the promise result...
-      if (isJsObject({})) {
       it('should throw an error on buffer overflow', (done) => {
-          createExtension([
+        PromiseWrapper.catchError(createExtension([
           chromeTimelineEvents.start('FunctionCall', 1234),
-          ], 'Tracing.bufferUsage').readPerfLog().then(null, (err) => {
+        ], 'Tracing.bufferUsage').readPerfLog(), (err) => {
           expect( () => {
             throw err;
           }).toThrowError('The DevTools trace buffer filled during the test!');
           done();
         });
       });
-      }
 
       it('should match chrome browsers', () => {
         expect(createExtension().supports({