Add a VmListener and IsolateTest class for running tests in other isolates.

test/utils.dart

Index: test/utils.dart
diff --git a/test/utils.dart b/test/utils.dart
index 17caceade76d789046cb438d4b71d0700521b302..f9b4075d70df066393024421491889b9a0561c3b 100644
--- a/test/utils.dart
+++ b/test/utils.dart
@@ -5,14 +5,72 @@
 library unittest.test.utils;
 
 import 'dart:async';
+import 'dart:collection';
 
+import 'package:unittest/src/live_test.dart';
+import 'package:unittest/src/remote_exception.dart';
+import 'package:unittest/src/state.dart';
 import 'package:unittest/unittest.dart';
 
+// The last state change detected via [expectStates].
+State lastState;
+
+/// Asserts that exactly [states] will be emitted via [liveTest.onStateChange].
+///
+/// The most recent emitted state is stored in [_lastState].
+void expectStates(LiveTest liveTest, Iterable<State> statesIter) {
+  var states = new Queue.from(statesIter);
+  liveTest.onStateChange.listen(expectAsync((state) {
+    lastState = state;
+    expect(state, equals(states.removeFirst()));
+  }, count: states.length, max: states.length));
+}
+
+/// Asserts that errors will be emitted via [liveTest.onError] that match
+/// [validators], in order.
+void expectErrors(LiveTest liveTest, Iterable<Function> validatorsIter) {
+  var validators = new Queue.from(validatorsIter);
+  liveTest.onError.listen(expectAsync((error) {
+    validators.removeFirst()(error.error);
+  }, count: validators.length, max: validators.length));
+}
+
+/// Asserts that [liveTest] will have a single failure with message `"oh no"`.
+void expectSingleFailure(LiveTest liveTest) {
+  expectStates(liveTest, [
+    const State(Status.running, Result.success),
+    const State(Status.complete, Result.failure)
+  ]);
+
+  expectErrors(liveTest, [(error) {
+    expect(lastState.status, equals(Status.complete));
+    expect(error, isTestFailure("oh no"));
+  }]);
+}
+
+/// Asserts that [liveTest] will have a single error, the string `"oh no"`.
+void expectSingleError(LiveTest liveTest) {
+  expectStates(liveTest, [
+    const State(Status.running, Result.success),
+    const State(Status.complete, Result.error)
+  ]);
+
+  expectErrors(liveTest, [(error) {
+    expect(lastState.status, equals(Status.complete));
+    expect(error, equals("oh no"));
+  }]);
+}
+
 /// Returns a matcher that matches a [TestFailure] with the given [message].
 Matcher isTestFailure(String message) => predicate(
     (error) => error is TestFailure && error.message == message,
     'is a TestFailure with message "$message"');
 
+/// Returns a matcher that matches a [RemoteException] with the given [message].
+Matcher isRemoteException(String message) => predicate(
+    (error) => error is RemoteException && error.message == message,
+    'is a RemoteException with message "$message"');
+
 /// Returns a [Future] that completes after pumping the event queue [times]
 /// times.
 ///