chore(npm): add shrinkwrap to lock down dependencies

We need to be able to build angular at older shas, without the lock file / shrinkwrap file
the dependencies will resolve differently on different machines and at different times.

This will help us avoid broken builds and hard to track down issues.

I had to manually edit this file after it was generated because `npm shrinkwrap` will install
optional dependencies as if they were hard dependencies.

See: https://github.com/npm/npm/issues/2679#issuecomment-37361236

My manual edit:

```
diff --git a/npm-shrinkwrap.json b/npm-shrinkwrap.json
index 756df44..dc157eb 100644
--- a/npm-shrinkwrap.json
+++ b/npm-shrinkwrap.json
@@ -3110,19 +3110,7 @@
         "chokidar": {
           "version": "0.8.1",
           "from": "https://registry.npmjs.org/chokidar/-/chokidar-0.8.1.tgz",
-          "resolved": "https://registry.npmjs.org/chokidar/-/chokidar-0.8.1.tgz",
-          "dependencies": {
-            "fsevents": {
-              "version": "0.1.6",
-              "from": "fsevents@0.1.6",
-              "resolved": "https://registry.npmjs.org/fsevents/-/fsevents-0.1.6.tgz"
-            },
-            "recursive-readdir": {
-              "version": "0.0.2",
-              "from": "recursive-readdir@0.0.2",
-              "resolved": "https://registry.npmjs.org/recursive-readdir/-/recursive-readdir-0.0.2.tgz"
-            }
-          }
+          "resolved": "https://registry.npmjs.org/chokidar/-/chokidar-0.8.1.tgz"
         },
         "glob": {
           "version": "3.2.9",
```

Additionally chokidar doesn't list the dependencies above as optional, but that will hopefully
be soon fixed: https://github.com/paulmillr/chokidar/pull/106

In the meantime the patch from the PR above needs to be applied to
node_modules/karma/node_modules/chokidar/package.json before running `npm shrinkwrap`

----

After this change is applied, angular core developers don't need to do anything differently,
except when updating dependencies we need to call `npm update && npm shrinkwrap --dev`
followed by reappling my patch above until npm's bug.

Closes #6653

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