fix(web-server): cache static files

Files in `karma/static/*` were always read from file system.
These files typically don’t change (except when developing Karma itself)
and so it makes sense to cache them in memory.
Especially `./static/context.html`, which is read for every test run.

The main reason for this change is that when using native events
(non-polling), watching can easily hit the EMFILE limit and then this
`fs.readFile()` can be super delayed. This was easily reproducible
problem in AngularJS test suite.

lib/middleware/common.js

@@ -26,13 +26,19 @@ var serve404 = function(response, path) {
 
 
 var createServeFile = function(fs, directory) {
+  var cache = Object.create(null);
+
   return function(filepath, response, transform, content) {
     var responseData;
 
     if (directory) {
       filepath = directory + filepath;
     }
 
+    if (!content && cache[filepath]) {
+      content = cache[filepath];
+    }
+
     // serve from cache
     if (content) {
       response.setHeader('Content-Type', mime.lookup(filepath, 'text/plain'));
@@ -51,6 +57,8 @@ var createServeFile = function(fs, directory) {
         return serve404(response, filepath);
       }
 
+      cache[filepath] = data.toString();
+
       response.setHeader('Content-Type', mime.lookup(filepath, 'text/plain'));
 
       // call custom transform fn to transform the data

test/unit/middleware/karma.spec.coffee

@@ -21,16 +21,17 @@ describe 'middleware.karma', ->
         'debug.html': mocks.fs.file(0, 'DEBUG\n%SCRIPTS%\n%X_UA_COMPATIBLE%')
         'karma.js': mocks.fs.file(0, 'root: %KARMA_URL_ROOT%, v: %KARMA_VERSION%')
 
-  serveFile = require('../../../lib/middleware/common').createServeFile fsMock, '/karma/static'
+  createServeFile = require('../../../lib/middleware/common').createServeFile
   createKarmaMiddleware = require('../../../lib/middleware/karma').create
 
-  handler = filesDeferred = nextSpy = response = null
+  handler = serveFile = filesDeferred = nextSpy = response = null
 
   beforeEach ->
     clientConfig = foo: 'bar'
     nextSpy = sinon.spy()
     response = new HttpResponseMock
     filesDeferred = q.defer()
+    serveFile = createServeFile fsMock, '/karma/static'
     handler = createKarmaMiddleware filesDeferred.promise, serveFile,
       '/base/path', '/__karma__/', clientConfig