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

tests/compiler/dart2js/cps_ir/expected/codeUnitAt_2.js

+// Expectation for test: 
+// // Bounds checking
+// foo(s) {
+//   var sum = 0;
+//   for (int i = 0; i < s.length; i++) sum += s.codeUnitAt(i);
+//   return sum;
+// }
+// main() {
+//   print(foo('ABC'));
+//   print(foo('Hello'));
+// }
+
+function() {
+  var v0 = "ABC".length, sum = 0, i = 0;
+  for (; i < v0; sum += "ABC".charCodeAt(i), ++i)
+    ;
+  P.print(sum);
+  sum = 0;
+  for (i = 0; i < "Hello".length; sum += "Hello".charCodeAt(i), ++i)
+    ;
+  P.print(sum);
+}