build(gulp): fix concurrency and caching issues in test.unit.js and test.unit.dart tasks

previously there was a chance of race conditions that could sporadically fail the build.

additionally runing a task via gulp.start or runSequence always reruns its dependencies, which meant that we were blowing away
the build.tools build and rebuilding everything from scratch even during the interactive/watch mode. This meant that the build
pipeline cache was destroyed on every change and we never got the benefit of incremental compilation

gulpfile.js

@@ -102,10 +102,16 @@ gulp.task('build/clean.docs', clean(gulp, gulpPlugins, {
 // ------------
 // transpile
 
-gulp.task('build/tree.dart', ['build.tools'], function() {
+gulp.task('build/tree.dart', ['build/clean.dart', 'build.tools'], function(done) {
+  runSequence('!build/tree.dart', done);
+});
+
+
+gulp.task('!build/tree.dart', function() {
   return angularBuilder.rebuildDartTree();
 });
 
+
 // ------------
 // pubspec
 
@@ -322,41 +328,68 @@ function getBrowsersFromCLI() {
   return [args.browsers?args.browsers:'DartiumWithWebPlatform']
 }
 
-gulp.task('test.unit.js', ['build/clean.js', 'broccoli.js.dev'], function (neverDone) {
 
-  function buildAndTest() {
+gulp.task('test.unit.js', ['build.js.dev'], function (neverDone) {
+
+  runSequence(
+    '!test.unit.js/karma-server',
+    '!test.unit.js/karma-run',
+    'check-format'
+  );
+
+  gulp.watch('modules/**', function() {
     runSequence(
-      'broccoli.js.dev',
-      'test.unit.js/karma-run'
+      '!broccoli.js.dev',
+      '!test.unit.js/karma-run',
+      'check-format'
     );
-  }
+  });
+});
 
-  karma.server.start({configFile: __dirname + '/karma-js.conf.js'});
 
-  gulp.watch('modules/**', buildAndTest);
+gulp.task('!test.unit.js/karma-server', function() {
+  karma.server.start({configFile: __dirname + '/karma-js.conf.js'});
 });
 
-gulp.task('test.unit.js/karma-run', function (done) {
-  karma.runner.run({configFile: __dirname + '/karma-js.conf.js'}, done);
+
+gulp.task('!test.unit.js/karma-run', function(done) {
+  karma.runner.run({configFile: __dirname + '/karma-js.conf.js'}, function(exitCode) {
+    // ignore exitCode, we don't want to fail the build in the interactive (non-ci) mode
+    // karma will print all test failures
+    done();
+  });
 });
 
+
 gulp.task('test.unit.dart', ['build/tree.dart'], function (done) {
-  function buildAndTest() {
+
+  runSequence(
+    '!test.unit.dart/karma-server',
+    '!test.unit.dart/karma-run'
+  );
+
+  gulp.watch('modules/angular2/**', function() {
     runSequence(
-      'build/tree.dart',
-      'test.unit.dart/karma-run'
+      '!build/tree.dart',
+      '!test.unit.dart/karma-run'
     );
-  }
-
-  karma.server.start({configFile: __dirname + '/karma-dart.conf.js'});
+  });
+});
 
-  gulp.watch('modules/angular2/**', buildAndTest);
+gulp.task('!test.unit.dart/karma-run', function (done) {
+  karma.runner.run({configFile: __dirname + '/karma-dart.conf.js'}, function(exitCode) {
+    // ignore exitCode, we don't want to fail the build in the interactive (non-ci) mode
+    // karma will print all test failures
+    done();
+  });
 });
 
-gulp.task('test.unit.dart/karma-run', function (done) {
-  karma.runner.run({configFile: __dirname + '/karma-dart.conf.js'}, done);
+
+gulp.task('!test.unit.dart/karma-server', function() {
+  karma.server.start({configFile: __dirname + '/karma-dart.conf.js'});
 });
 
+
 gulp.task('test.unit.js/ci', function (done) {
   karma.server.start({configFile: __dirname + '/karma-js.conf.js',
     singleRun: true, reporters: ['dots'], browsers: getBrowsersFromCLI()}, done);
@@ -538,12 +571,16 @@ gulp.task('!build.tools', function() {
   return mergedStream;
 });
 
-gulp.task('broccoli.js.dev', ['build.tools'], function() {
+gulp.task('broccoli.js.dev', ['build.tools'], function(done) {
+  runSequence('!broccoli.js.dev', done);
+});
+
+gulp.task('!broccoli.js.dev', function() {
   return angularBuilder.rebuildBrowserDevTree();
 });
 
 
-gulp.task('build.js.dev', function(done) {
+gulp.task('build.js.dev', ['build/clean.js'], function(done) {
   runSequence(
     'broccoli.js.dev',
     'build/checkCircularDependencies',