Add "mock" compilation feature to dart2js and use it to improve test coverage.

dart/frog/leg/apiimpl.dart

 // Copyright (c) 2012, 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('leg_apiimpl');
 
 #import('leg.dart', prefix: 'leg');
 #import('elements/elements.dart', prefix: 'leg');
 #import('tree/tree.dart', prefix: 'leg');
 #import('ssa/tracer.dart', prefix: 'ssa');
 #import('../lang.dart', prefix: 'frog');
 #import('api.dart');
 #import('../../lib/uri/uri.dart');
 
 class Compiler extends leg.Compiler {
   ReadUriFromString provider;
   DiagnosticHandler handler;
   Uri libraryRoot;
+  List<String> options;
+  bool mockableLibraryUsed = false;
 
-  Compiler(this.provider, this.handler, this.libraryRoot)
+  Compiler(this.provider, this.handler, this.libraryRoot, this.options)
     : super.withCurrentDirectory(null, tracer: new ssa.HTracer());
 
   leg.LibraryElement scanBuiltinLibrary(String filename) {
     Uri uri = libraryRoot.resolve(filename);
     leg.LibraryElement library = scanner.loadLibrary(uri, null);
     return library;
   }
 
   void log(message) {
     handler(null, null, null, message, false);
@@ skipping 12 matching lines @@
       cancel("${uri}: $exception", node: node);
     }
     frog.SourceFile sourceFile = new frog.SourceFile(uri.toString(), text);
     return new leg.Script(uri, sourceFile);
   }
 
   translateDartUri(Uri uri, leg.ScriptTag node) {
     String uriName = uri.toString();
     // TODO(ahe): Clean this up.
     if (uriName == 'dart:dom') {
+      mockableLibraryUsed = true;
       return libraryRoot.resolve('../../../client/dom/frog/dom_frog.dart');
     } else if (uriName == 'dart:html') {
+      mockableLibraryUsed = true;
       return libraryRoot.resolve('../../../client/html/frog/html_frog.dart');
     } else if (uriName == 'dart:json') {
       return libraryRoot.resolve('../../../lib/json/json.dart');
     } else if (uriName == 'dart:isolate') {
       return libraryRoot.resolve('../../../lib/isolate/isolate_leg.dart');
     } else if (uriName == 'dart:io') {
+      mockableLibraryUsed = true;
       return libraryRoot.resolve('io.dart');
     } else if (uriName == 'dart:utf') {
       return libraryRoot.resolve('../../../lib/utf/utf.dart');
     } else if (uriName == 'dart:uri') {
       return libraryRoot.resolve('../../../lib/uri/uri.dart');
     }
     reportError(node, "library not found $uriName");
   }
 
   bool run(Uri uri) {
     bool success = super.run(uri);
     for (final task in tasks) {
       log('${task.name} took ${task.timing}msec');
     }
     return success;
   }
 
   void reportDiagnostic(leg.SourceSpan span, String message, bool fatal) {
     if (span === null) {
       handler(null, null, null, message, fatal);
     } else {
       handler(span.uri, span.begin, span.end, message, fatal);
     }
   }
+
+  bool get isMockCompilation() {
+    return mockableLibraryUsed
+      && (options.indexOf('--allow-mock-compilation') !== -1);
+  }
 }