Implement search for PrefixElement.

R=brianwilkerson@google.com
BUG=

Review URL: https://codereview.chromium.org/2535173004 .

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

@@ -60,6 +60,8 @@ class Search {
       return _searchReferences_Local(element, (n) => n is Block);
     } else if (kind == ElementKind.PARAMETER) {
       return _searchReferences_Parameter(element);
+    } else if (kind == ElementKind.PREFIX) {
+      return _searchReferences_Prefix(element);
     } else if (kind == ElementKind.TYPE_PARAMETER) {
       return _searchReferences_Local(
           element, (n) => n.parent is CompilationUnit);
@@ -196,6 +198,27 @@ class Search {
     }));
     return results;
   }
+
+  Future<List<SearchResult>> _searchReferences_Prefix(
+      PrefixElement element) async {
+    // Search only in drivers to which the library with the prefix was added.
+    String path = element.source.fullName;
+    if (!_driver.addedFiles.contains(path)) {
+      return const <SearchResult>[];
+    }
+
+    List<SearchResult> results = <SearchResult>[];
+    LibraryElement libraryElement = element.library;
+    for (CompilationUnitElement unitElement in libraryElement.units) {
+      String unitPath = unitElement.source.fullName;
+      AnalysisResult unitAnalysisResult = await _driver.getResult(unitPath);
+      _LocalReferencesVisitor visitor =
+          new _LocalReferencesVisitor(element, unitElement);
+      unitAnalysisResult.unit.accept(visitor);
+      results.addAll(visitor.results);
+    }
+    return results;
+  }
 }
 
 /**

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

@@ -38,6 +38,7 @@ class ExpectedResult {
         result.isResolved == this.isResolved &&
         result.isQualified == this.isQualified &&
         result.offset == this.offset &&
+        result.length == this.length &&
         result.enclosingElement == this.enclosingElement;
   }
 
@@ -510,6 +511,34 @@ main() {
     await _verifyReferences(element, expected);
   }
 
+  test_searchReferences_PrefixElement() async {
+    String partCode = r'''
+part of my_lib;
+ppp.Future c;
+''';
+    provider.newFile(_p('$testProject/my_part.dart'), partCode);
+    await _resolveTestUnit('''
+library my_lib;
+import 'dart:async' as ppp;
+part 'my_part.dart';
+main() {
+  ppp.Future a;
+  ppp.Stream b;
+}
+''');
+    PrefixElement element = _findElementAtString('ppp;');
+    Element a = _findElement('a');
+    Element b = _findElement('b');
+    Element c = findChildElement(testLibraryElement, 'c');
+    var expected = [
+      _expectId(a, SearchResultKind.REFERENCE, 'ppp.Future'),
+      _expectId(b, SearchResultKind.REFERENCE, 'ppp.Stream'),
+      new ExpectedResult(c, SearchResultKind.REFERENCE,
+          partCode.indexOf('ppp.Future c'), 'ppp'.length)
+    ];
+    await _verifyReferences(element, expected);
+  }
+
   test_searchReferences_PropertyAccessorElement_getter() async {
     await _resolveTestUnit('''
 class A {