Add a new flag to dart2js: --trust-type-annotations and implement it in the types inferrer.

sdk/lib/_internal/compiler/implementation/dart2js.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library dart2js;
 
 import 'dart:async';
 import 'dart:collection' show Queue, LinkedHashMap;
 import 'dart:io';
 import 'dart:uri';
@@ skipping 194 matching lines @@
     new OptionHandler('--force-strip=.*', setStrip),
     // TODO(ahe): Remove the --no-colors option.
     new OptionHandler('--disable-diagnostic-colors',
                       (_) => diagnosticHandler.enableColors = false),
     new OptionHandler('--enable-diagnostic-colors',
                       (_) => diagnosticHandler.enableColors = true),
     new OptionHandler('--enable[_-]checked[_-]mode|--checked',
                       (_) => passThrough('--enable-checked-mode')),
     new OptionHandler('--enable-concrete-type-inference',
                       (_) => passThrough('--enable-concrete-type-inference')),
+    new OptionHandler('--trust-type-annotations',
+                      (_) => passThrough('--trust-type-annotations')),
     new OptionHandler(r'--help|/\?|/h', (_) => wantHelp = true),
     new OptionHandler('--package-root=.+|-p.+', setPackageRoot),
     new OptionHandler('--disallow-unsafe-eval', passThrough),
     new OptionHandler('--analyze-all', passThrough),
     new OptionHandler('--analyze-only', setAnalyzeOnly),
     new OptionHandler('--analyze-signatures-only', passThrough),
     new OptionHandler('--disable-native-live-type-analysis', passThrough),
     new OptionHandler('--reject-deprecated-language-features', passThrough),
     new OptionHandler('--report-sdk-use-of-deprecated-language-features',
                       passThrough),
@@ skipping 289 matching lines @@
       print(trace);
     } finally {
       exit(253); // 253 is recognized as a crash by our test scripts.
     }
   }
 }
 
 void main() {
   mainWithErrorHandler(new Options());
 }