refactor(schematics): add tests for component walker (#13106)

* The `ComponentWalker` is a major part of the update schematics and we should verify that external resource files are resolved properly. All test-cases currently just use inline stylesheets or templates.
* Removes the `color` helper function.
* Removes the superfluous utility function for retrieving a `StringLiteral`'s text content.

package.json

@@ -65,6 +65,7 @@
     "@types/jasmine": "^2.8.8",
     "@types/merge2": "^0.3.30",
     "@types/minimist": "^1.2.0",
+    "@types/mock-fs": "^3.6.30",
     "@types/node": "^7.0.21",
     "@types/parse5": "^5.0.0",
     "@types/run-sequence": "^0.0.29",
@@ -112,6 +113,7 @@
     "magic-string": "^0.22.4",
     "minimatch": "^3.0.4",
     "minimist": "^1.2.0",
+    "mock-fs": "^4.7.0",
     "moment": "^2.18.1",
     "node-sass": "^4.9.3",
     "parse5": "^5.0.0",