feat(dart/transform): Improve constant evaluation

Use `package:analyzer`'s `ConstantEvaluator` to read from the AST.
This cleanly builds values for us from adjacent strings, interpolations,
etc.

modules/angular2/test/transform/directive_processor/all_tests.dart

@@ -39,6 +39,9 @@ void allTests() {
 
   _testNgDeps(
       'should inline `templateUrl` values.', 'url_expression_files/hello.dart');
+
+  _testNgDeps('should inline `templateUrl`s expressed as adjacent strings.',
+      'split_url_expression_files/hello.dart');
 }
 
 void _testNgDeps(String name, String inputPath,

modules/angular2/test/transform/directive_processor/split_url_expression_files/expected/hello.ng_deps.dart

@@ -0,0 +1,20 @@
+library examples.src.hello_world.index_common_dart.ng_deps.dart;
+
+import 'hello.dart';
+import 'package:angular2/angular2.dart'
+    show bootstrap, Component, Directive, View, NgElement;
+
+var _visited = false;
+void initReflector(reflector) {
+  if (_visited) return;
+  _visited = true;
+  reflector
+    ..registerType(HelloCmp, {
+      'factory': () => new HelloCmp(),
+      'parameters': const [],
+      'annotations': const [
+        const Component(selector: 'hello-app'),
+        const View(template: r'''{{greeting}}''')
+      ]
+    });
+}

modules/angular2/test/transform/directive_processor/split_url_expression_files/hello.dart

@@ -0,0 +1,8 @@
+library examples.src.hello_world.index_common_dart;
+
+import 'package:angular2/angular2.dart'
+    show bootstrap, Component, Directive, View, NgElement;
+
+@Component(selector: 'hello-app')
+@View(templateUrl: 'templ' 'ate.html')
+class HelloCmp {}

modules/angular2/test/transform/directive_processor/split_url_expression_files/template.html

@@ -0,0 +1 @@
+{{greeting}}