Add async tests from prototype

tests/012_async.ts

@@ -0,0 +1,11 @@
+// Check that we can use the async keyword.
+async function main() {
+  await new Promise((resolve, reject) => {
+    console.log("2");
+    setTimeout(resolve, 100);
+  });
+  console.log("3");
+}
+
+console.log("1");
+main();

tests/012_async.ts.out

@@ -0,0 +1,3 @@
+1
+2
+3

tests/async_error.ts

@@ -0,0 +1,9 @@
+
+console.log("hello");
+const foo = async () => {
+  console.log("before error");
+  throw Error("error");
+}
+
+foo();
+console.log("world");

tests/async_error.ts.out

@@ -0,0 +1,10 @@
+hello
+before error
+Error: error
+    at foo ([WILDCARD]tests/async_error.ts:5:9)
+    at eval ([WILDCARD]tests/async_error.ts:8:1)
+    at eval (<anonymous>)
+    at execute (deno/js/runtime.ts:[WILDCARD])
+    at FileModule.compileAndRun (deno/js/runtime.ts:[WILDCARD])
+    at denoMain (deno/js/main.ts:[WILDCARD])
+    at deno_main.js:1:1