Add AnalysisDriver.getUnitElementSignature() for checking without resynthesizing.

pkg/analyzer/test/src/dart/analysis/driver_test.dart

 // Copyright (c) 2016, 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.
 
 library analyzer.test.driver;
 
 import 'dart:async';
 import 'dart:convert';
 
 import 'package:analyzer/dart/ast/ast.dart';
@@ skipping 1538 matching lines @@
   }
 
   test_getUnitElement_notDart() async {
     var path = _p('/test.txt');
     provider.newFile(path, 'class A {}');
     UnitElementResult unitResult = await driver.getUnitElement(path);
     expect(unitResult, isNotNull);
     expect(unitResult.element.types.map((e) => e.name), ['A']);
   }
 
+  test_getUnitElementSignature() async {
+    var a = _p('/test/lib/a.dart');
+
+    provider.newFile(a, 'foo() {}');
+
+    String signature = await driver.getUnitElementSignature(a);
+    expect(signature, isNotNull);
+
+    UnitElementResult unitResult = await driver.getUnitElement(a);
+    expect(unitResult.path, a);
+    expect(unitResult.signature, signature);
+
+    provider.updateFile(a, 'bar() {}');
+    driver.changeFile(a);
+
+    String signature2 = await driver.getUnitElementSignature(a);
+    expect(signature2, isNotNull);
+    expect(signature2, isNot(signature));
+  }
+
   test_hasFilesToAnalyze() async {
     // No files yet, nothing to analyze.
     expect(driver.hasFilesToAnalyze, isFalse);
 
     // Add a new file, it should be analyzed.
     addTestFile('main() {}', priority: false);
     expect(driver.hasFilesToAnalyze, isTrue);
 
     // Wait for idle, nothing to do.
     await scheduler.waitForIdle();
@@ skipping 767 matching lines @@
    * Return the [provider] specific path for the given Posix [path].
    */
   String _p(String path) => provider.convertPath(path);
 
   static String _md5(String content) {
     return hex.encode(md5.convert(UTF8.encode(content)).bytes);
   }
 }
 
 class _SourceMock extends TypedMock implements Source {}