Do nothing if [changeFile] reported, but the file content is the same.

R=brianwilkerson@google.com, paulberry@google.com
BUG=

Review URL: https://codereview.chromium.org/2458853003 .

pkg/analyzer/lib/src/dart/analysis/driver.dart

@@ -208,12 +208,6 @@ class AnalysisDriver {
    * "analyzing" and an analysis result is produced for every added file prior
    * to the next time the analysis state transitions to "idle".
    *
-   * At least one analysis result is produced for every file passed to
-   * [addFile] or [changeFile] prior to the next time the analysis state
-   * transitions to "idle", unless the file is later removed from analysis
-   * using [removeFile]. Analysis results for other files are produced only if
-   * the changes affect analysis results of other files.
-   *
    * More than one result might be produced for the same file, even if the
    * client does not change the state of the files.
    *
@@ -305,6 +299,11 @@ class AnalysisDriver {
    * The [path] must be absolute and normalized.
    *
    * The results of analysis are eventually produced by the [results] stream.
+   *
+   * Causes the analysis state to transition to "analyzing" (if it is not in
+   * that state already). At least one analysis result will be produced the
+   * file prior to the next time the analysis state transitions to "idle",
+   * unless the file is later removed from analysis using [removeFile].
    */
   void addFile(String path) {
     _explicitFiles.add(path);
@@ -326,6 +325,11 @@ class AnalysisDriver {
    * into the current file state prior to the next time the analysis state
    * transitions to "idle".
    *
+   * If the file content is the same, no new results will be produced because
+   * of this notification, including no result for the file itself. Otherwise,
+   * one or more results will be produced - for the file itself and other
+   * files that that change in the file might affect.
+   *
    * Invocation of this method will not prevent a [Future] returned from
    * [getResult] from completing with a result, but the result is not
    * guaranteed to be consistent with the new current file state after this
@@ -681,16 +685,22 @@ class AnalysisDriver {
    */
   void _verifyApiSignatureOfChangedFile(String path) {
     _logger.run('Verify API signature of $path', () {
+      String oldContentHash = _fileContentHashMap[path];
       String oldSignature = _fileApiSignatureMap[path];
       // Compute the new API signature.
       // _File.forResolution() also updates the content hash in the cache.
       Source source = _sourceForPath(path);
       _File newFile = new _File.forResolution(this, source);
-      String newSignature = newFile.unlinked.apiSignature;
+      // If the file content hash is the same, we don't need analyzing it.
+      if (newFile.contentHash == oldContentHash) {
+        _filesToAnalyze.remove(path);
+        return;
+      }
       // If the old API signature is not null, then the file was used to
       // compute at least one dependency signature. If the new API signature
       // is different, then potentially all dependency signatures and
       // resolution results are invalid.
+      String newSignature = newFile.unlinked.apiSignature;
       if (oldSignature != null && oldSignature != newSignature) {
         _logger.writeln('API signatures mismatch found for $newFile');
         _dependencySignatureMap.clear();

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

@@ -77,7 +77,7 @@ class DriverTest {
           })
         ], null, provider),
         new AnalysisOptionsImpl()..strongMode = true);
-    driver.status.lastWhere((status) {
+    driver.status.listen((status) {
       allStatuses.add(status);
       if (status.isIdle) {
         idleStatusMonitor.notify();
@@ -141,6 +141,30 @@ var A = B;
     }
   }
 
+  test_changeFile_noContentChange_noNewResult() async {
+    _addTestFile('main() {}', priority: true);
+
+    // Initial analysis.
+    await _waitForIdle();
+    expect(allResults, hasLength(1));
+
+    // Update the file, but don't notify the driver.
+    // Don't update the file in the file system.
+    // But tell the driver the the file was changed.
+    // The driver should eventually check the file and ignore.
+    allStatuses.clear();
+    allResults.clear();
+    driver.changeFile(testFile);
+
+    // The driver switched to analysis and back to idle.
+    await _waitForIdle();
+    expect(allStatuses, hasLength(2));
+    expect(allStatuses.map((status) => status.isAnalyzing), [true, false]);
+
+    // No new results.
+    expect(allResults, isEmpty);
+  }
+
   test_changeFile_selfConsistent() async {
     var a = _p('/test/lib/a.dart');
     var b = _p('/test/lib/b.dart');