feat(compiler): assert that Component.style is an array

Part of #7481 (effort to improve error messages)

Closes #7559

modules/angular2/src/compiler/assertions.dart

@@ -0,0 +1,3 @@
+library angular2.core.util.asserions;
+
+void assertArrayOfStrings(String identifier, Object value) {}

modules/angular2/src/compiler/assertions.ts

@@ -0,0 +1,16 @@
+import {isArray, isString, isBlank, assertionsEnabled} from '../facade/lang';
+import {BaseException} from '../facade/exceptions';
+
+export function assertArrayOfStrings(identifier: string, value: any) {
+  if (!assertionsEnabled() || isBlank(value)) {
+    return;
+  }
+  if (!isArray(value)) {
+    throw new BaseException(`Expected '${identifier}' to be an array of strings.`);
+  }
+  for (var i = 0; i < value.length; i += 1) {
+    if (!isString(value[i])) {
+      throw new BaseException(`Expected '${identifier}' to be an array of strings.`);
+    }
+  }
+}

modules/angular2/src/compiler/runtime_metadata.ts

@@ -20,6 +20,7 @@ import {reflector} from 'angular2/src/core/reflection/reflection';
 import {Injectable, Inject, Optional} from 'angular2/src/core/di';
 import {PLATFORM_DIRECTIVES, PLATFORM_PIPES} from 'angular2/src/core/platform_directives_and_pipes';
 import {MODULE_SUFFIX} from './util';
+import {assertArrayOfStrings} from './assertions';
 import {getUrlScheme} from 'angular2/src/compiler/url_resolver';
 
 @Injectable()
@@ -41,9 +42,11 @@ export class RuntimeMetadataResolver {
       var changeDetectionStrategy = null;
 
       if (dirMeta instanceof md.ComponentMetadata) {
+        assertArrayOfStrings('styles', dirMeta.styles);
         var cmpMeta = <md.ComponentMetadata>dirMeta;
         moduleUrl = calcModuleUrl(directiveType, cmpMeta);
         var viewMeta = this._viewResolver.resolve(directiveType);
+        assertArrayOfStrings('styles', viewMeta.styles);
         templateMeta = new cpl.CompileTemplateMetadata({
           encapsulation: viewMeta.encapsulation,
           template: viewMeta.template,