Add --analyze-signatures-only option to dart2js

sdk/lib/_internal/compiler/implementation/resolution/members.dart

Index: sdk/lib/_internal/compiler/implementation/resolution/members.dart
diff --git a/sdk/lib/_internal/compiler/implementation/resolution/members.dart b/sdk/lib/_internal/compiler/implementation/resolution/members.dart
index 1ff6f484cba8314292db2f3b4251514eb9d722a9..6fcd842e88176b51c5e399d4e1dad1761de5c845 100644
--- a/sdk/lib/_internal/compiler/implementation/resolution/members.dart
+++ b/sdk/lib/_internal/compiler/implementation/resolution/members.dart
@@ -363,7 +363,9 @@ class ResolverTask extends CompilerTask {
   }
 
   void visitBody(ResolverVisitor visitor, Statement body) {
-    visitor.visit(body);
+    if (!compiler.analyzeSignaturesOnly) {
+      visitor.visit(body);
+    }
   }
 
   void resolveConstructorImplementation(FunctionElement constructor,
@@ -434,6 +436,8 @@ class ResolverTask extends CompilerTask {
             MessageKind.TOP_LEVEL_VARIABLE_DECLARED_STATIC);
     }
     ResolverVisitor visitor = visitorFor(element);
+    // TODO(johnniwinther): Avoid analyzing initializers if
+    // [Compiler.analyzeSignaturesOnly] is set.
     initializerDo(tree, visitor.visit);
 
     if (Elements.isStaticOrTopLevelField(element)) {