Add --analyze-signatures-only option to dart2js

tests/compiler/dart2js/analyze_only_test.dart

Index: tests/compiler/dart2js/analyze_only_test.dart
diff --git a/tests/compiler/dart2js/analyze_only_test.dart b/tests/compiler/dart2js/analyze_only_test.dart
index f17aed48b1d02be16c8c524e56c83e4e34c995f3..fad5eb53cf5e2c0ae437083ec4ec6353b411cafe 100644
--- a/tests/compiler/dart2js/analyze_only_test.dart
+++ b/tests/compiler/dart2js/analyze_only_test.dart
@@ -108,6 +108,17 @@ main() {
     });
 
   runCompiler(
+    """main() {
+         Foo foo; // Unresolved and analyzed.
+       }""",
+    ['--analyze-only', '--analyze-signatures-only'],
+    (String code, List errors, List warnings) {
+      Expect.isNull(code);
+      Expect.isTrue(errors.isEmpty);
+      Expect.isTrue(warnings.isEmpty);
+    });
+
+  runCompiler(
     "Foo foo; // Unresolved and analyzed.",
     ['--analyze-only', '--analyze-all'],
     (String code, List errors, List warnings) {