Allow multiple dart <script> tags in .sky files

sky/tests/modules/multiple-scripts-import.sky

+<sky>
+<import src='resources/multiple-scripts-child.sky' as='child'></import>
+<script>
+import "../resources/third_party/unittest/unittest.dart";
+import "../resources/unit.dart";
+
+import "dart:sky";
+import "dart:sky.internals";
+
+void main () {
+  initUnit();
+
+  test('multiple libraries should combine using "as"', () {
+    expect(child.one, 'one');
+    expect(child.two, 'two');
+  });
+
+  test('multiple libraries should each init', () {
+    expect(child.oneInit, true);
+    expect(child.twoInit, true);
+  });
+
+  test('conflicting names should be ignored', () {
+    expect(() => child.conflict, throwsNoSuchMethodError);
+  });
+}
+</script>
+</sky>