Add --analyze-signatures-only option to dart2js

tests/compiler/dart2js/analyze_only_test.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.
 
 // Smoke test of the dart2js compiler API.
 library analyze_only;
 
 import 'dart:async';
 import 'dart:uri';
 
@@ skipping 90 matching lines @@
        }""",
     ['--analyze-only'],
     (String code, List errors, List warnings) {
       Expect.isNull(code);
       Expect.isTrue(errors.isEmpty);
       Expect.equals(1, warnings.length);
       Expect.equals('Warning: cannot resolve type Foo', warnings[0].toString());
     });
 
   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) {
       Expect.isNull(code);
       Expect.isTrue(errors.isEmpty);
       Expect.equals('Warning: cannot resolve type Foo', warnings[0].toString());
     });
 
   runCompiler(
     """Foo foo; // Unresolved and analyzed.
        main() {}""",
     ['--analyze-only', '--analyze-all'],
     (String code, List errors, List warnings) {
       Expect.isNull(code);
       Expect.isTrue(errors.isEmpty);
       Expect.equals(1, warnings.length);
       Expect.equals('Warning: cannot resolve type Foo', warnings[0].toString());
     });
 }