Add MemoryByteStore and use it in tests.

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

Index: pkg/analyzer/test/src/dart/analysis/driver_test.dart
diff --git a/pkg/analyzer/test/src/dart/analysis/driver_test.dart b/pkg/analyzer/test/src/dart/analysis/driver_test.dart
index ccdcc9223baf1228ba20f462feb9980e3e3cbfe1..3b7d74d4a6b7d13c15632afd34a17b6934e71478 100644
--- a/pkg/analyzer/test/src/dart/analysis/driver_test.dart
+++ b/pkg/analyzer/test/src/dart/analysis/driver_test.dart
@@ -53,7 +53,7 @@ class AnalysisDriverSchedulerTest {
   static final MockSdk sdk = new MockSdk();
 
   final MemoryResourceProvider provider = new MemoryResourceProvider();
-  final ByteStore byteStore = new _TestByteStore();
+  final ByteStore byteStore = new MemoryByteStore();
   final FileContentOverlay contentOverlay = new FileContentOverlay();
 
   final StringBuffer logBuffer = new StringBuffer();
@@ -220,7 +220,7 @@ class AnalysisDriverTest {
   static final MockSdk sdk = new MockSdk();
 
   final MemoryResourceProvider provider = new MemoryResourceProvider();
-  final ByteStore byteStore = new _TestByteStore();
+  final ByteStore byteStore = new MemoryByteStore();
   final FileContentOverlay contentOverlay = new FileContentOverlay();
 
   final StringBuffer logBuffer = new StringBuffer();
@@ -1240,17 +1240,3 @@ class _Monitor {
     }
   }
 }
-
-class _TestByteStore implements ByteStore {
-  final map = <String, List<int>>{};
-
-  @override
-  List<int> get(String key) {
-    return map[key];
-  }
-
-  @override
-  void put(String key, List<int> bytes) {
-    map[key] = bytes;
-  }
-}