Script-based imports should basically work

This CL adds the beginning of a new module loading system that matches the
conceptual module in the spec. I've writed it up to the import API on
AbstractModule. In a future CL, I'll replace the old module loading system with
this one.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/720903002

engine/src/flutter/tests/modules/script-import-expected.txt

@@ -0,0 +1 @@
+PASS: pass.sky succesfully exported this string.

engine/src/flutter/tests/modules/script-import.sky

@@ -0,0 +1,5 @@
+<script>
+module.import("resources/pass.sky").then(function(hello) {
+  internals.notifyTestComplete(hello);
+});
+</script>