Add an "api signature" to summaries.

pkg/analyzer/test/src/summary/linker_test.dart

Index: pkg/analyzer/test/src/summary/linker_test.dart
diff --git a/pkg/analyzer/test/src/summary/linker_test.dart b/pkg/analyzer/test/src/summary/linker_test.dart
index 674ce9b7c3fbc9e4ce7c99dc640e0acf6d7849af..196fb04f22bd76c6165d0c26153c6f0f1297db2d 100644
--- a/pkg/analyzer/test/src/summary/linker_test.dart
+++ b/pkg/analyzer/test/src/summary/linker_test.dart
@@ -43,6 +43,24 @@ class LinkerUnitTest extends SummaryLinkerTest {
     return linker.getLibrary(Uri.parse(uri));
   }
 
+  void test_apiSignature_apiChanges() {
+    var bundle0 =
+        createPackageBundle('f(int i) { print(i); }', path: '/test.dart');
+    var bundle1 =
+        createPackageBundle('f(String s) { print(s); }', path: '/test.dart');
+    expect(bundle0.apiSignature, isNotEmpty);
+    expect(bundle1.apiSignature, isNotEmpty);
+    expect(bundle0.apiSignature, isNot(bundle1.apiSignature));
+  }
+
+  void test_apiSignature_localChanges() {
+    var bundle0 = createPackageBundle('f() { print(0); }', path: '/test.dart');
+    var bundle1 = createPackageBundle('f() { print(1); }', path: '/test.dart');
+    expect(bundle0.apiSignature, isNotEmpty);
+    expect(bundle1.apiSignature, isNotEmpty);
+    expect(bundle0.apiSignature, bundle1.apiSignature);
+  }
+
   void test_baseClass_genericWithAccessor() {
     createLinker('''
 class B<T> {