dart2js: Support mock compilation in the program builder.

pkg/compiler/lib/src/js_emitter/program_builder/program_builder.dart

Index: pkg/compiler/lib/src/js_emitter/program_builder/program_builder.dart
diff --git a/pkg/compiler/lib/src/js_emitter/program_builder/program_builder.dart b/pkg/compiler/lib/src/js_emitter/program_builder/program_builder.dart
index deadcb73e45f98b3adc99294156ef817ae67a765..d41b5e499dd4694bcabe716ff843cfda1b6b3bd0 100644
--- a/pkg/compiler/lib/src/js_emitter/program_builder/program_builder.dart
+++ b/pkg/compiler/lib/src/js_emitter/program_builder/program_builder.dart
@@ -208,6 +208,8 @@ class ProgramBuilder {
   }
 
   js.Statement _buildInvokeMain() {
+    if (_compiler.isMockCompilation) return js.js.comment("Mock compilation");
+
     MainCallStubGenerator generator =
         new MainCallStubGenerator(_compiler, backend, backend.emitter);
     return generator.generateInvokeMain();