Add --analyze-signatures-only option to dart2js

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

Index: sdk/lib/_internal/compiler/implementation/dart2js.dart
diff --git a/sdk/lib/_internal/compiler/implementation/dart2js.dart b/sdk/lib/_internal/compiler/implementation/dart2js.dart
index c76c45f1a4a176adf1db478c5276e4c81f8e2fbe..7be2db8c25176928cfbf6536e065d8e84ef61ab2 100644
--- a/sdk/lib/_internal/compiler/implementation/dart2js.dart
+++ b/sdk/lib/_internal/compiler/implementation/dart2js.dart
@@ -199,6 +199,7 @@ void compile(List<String> argv) {
     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',
@@ -403,6 +404,10 @@ Supported options:
   --analyze-only
     Analyze but do not generate code.
 
+  --analyze-signatures-only
+    Skip analysis of method bodies and field initializers. This option implies
+    --analyze-only.
+
   --minify
     Generate minified output.