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

dart/frog/leg/emitter.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.
 
 /**
  * A function element that represents a closure call. The signature is copied
  * from the given element.
  */
 class ClosureInvocationElement extends FunctionElement {
   ClosureInvocationElement(SourceString name,
@@ skipping 635 matching lines @@
     String currentIsolate = "${namer.CURRENT_ISOLATE}";
     String mainEnsureGetter = '';
     // Since we pass the closurized version of the main method to
     // the isolate method, we must make sure that it exists.
     if (!compiler.universe.staticFunctionsNeedingGetter.contains(appMain)) {
       String invocationName =
           "${namer.closureInvocationName(Selector.INVOCATION_0)}";
       mainEnsureGetter = "$mainAccess.$invocationName = $mainAccess";
     }
 
-  // TODO(ngeoffray): These globals are currently required by the isolate
-  // library, but since leg already generates code on an Isolate object, they
-  // are not really needed. We should remove them once Leg replaces Frog.
+    // TODO(ngeoffray): These globals are currently required by the isolate
+    // library, but since leg already generates code on an Isolate object, they
+    // are not really needed. We should remove them once Leg replaces Frog.
     return """
 var \$globalThis = $currentIsolate;
 var \$globalState;
 var \$globals;
 function \$static_init(){};
 
 function \$initGlobals(context) {
   context.isolateStatics = new ${namer.ISOLATE}();
 }
 function \$setGlobals(context) {
   $currentIsolate = context.isolateStatics;
   \$globalThis = $currentIsolate;
 }
 $mainEnsureGetter
 ${namer.isolateAccess(isolateMain)}($mainAccess);""";
   }
 
+  emitMain(StringBuffer buffer) {
+    if (compiler.isMockCompilation) return;
+    Element main = compiler.mainApp.find(Compiler.MAIN);
+    if (compiler.isolateLibrary != null) {
+      Element isolateMain =
+        compiler.isolateLibrary.find(Compiler.START_ROOT_ISOLATE);
+      buffer.add(buildIsolateSetup(main, isolateMain));
+    } else {
+      buffer.add('${namer.isolateAccess(main)}();\n');
+    }
+  }
+
   String assembleProgram() {
     measure(() {
       StringBuffer buffer = new StringBuffer();
       buffer.add('function ${namer.ISOLATE}() {');
       emitStaticNonFinalFieldInitializations(buffer);
       buffer.add('}\n\n');
       emitClasses(buffer);
       emitStaticFunctions(buffer);
       emitStaticFunctionGetters(buffer);
       emitCompileTimeConstants(buffer);
       emitStaticFinalFieldInitializations(buffer);
       nativeEmitter.emitDynamicDispatchMetadata(buffer);
       buffer.add('var ${namer.CURRENT_ISOLATE} = new ${namer.ISOLATE}();\n');
-      Element main = compiler.mainApp.find(Compiler.MAIN);
-      if (compiler.isolateLibrary != null) {
-        Element isolateMain =
-            compiler.isolateLibrary.find(Compiler.START_ROOT_ISOLATE);
-        buffer.add(buildIsolateSetup(main, isolateMain));
-      } else {
-        buffer.add('${namer.isolateAccess(main)}();\n');
-      }
+      emitMain(buffer);
       compiler.assembledCode = buffer.toString();
     });
     return compiler.assembledCode;
   }
 }