cpsir unittests: move all unittests into individual files and test runners.

tests/compiler/dart2js/cps_ir/expected/closures_6.js

+// Expectation for test: 
+// main() {
+//   var x = 122;
+//   var a = () => x;
+//   x = x + 1;
+//   print(a());
+//   return a;
+// }
+
+function() {
+  var _box_0 = {}, a = new V.main_closure(_box_0), line;
+  _box_0.x = 122;
+  ++_box_0.x;
+  line = H.S(a.call$0());
+  if (typeof dartPrint == "function")
+    dartPrint(line);
+  else if (typeof console == "object" && typeof console.log != "undefined")
+    console.log(line);
+  else if (!(typeof window == "object")) {
+    if (!(typeof print == "function"))
+      throw "Unable to print message: " + String(line);
+    print(line);
+  }
+  return a;
+}