[modules] Do basic linking.

test/mjsunit/modules-imports2.js

+// Copyright 2016 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+//
+// MODULE
+
+let get_x = () => x;
+
+assertEquals(1, x);
+assertEquals(1, (() => x)());
+assertEquals(1, eval('x'));
+assertEquals(1, get_x());
+
+assertThrows(() => x = 666, TypeError);
+assertEquals(1, x);
+assertEquals(1, (() => x)());
+assertEquals(1, eval('x'));
+assertEquals(1, get_x());
+
+set_x("foo");
+assertEquals("foo", x);
+assertEquals("foo", (() => x)());
+assertEquals("foo", eval('x'));
+assertEquals("foo", get_x());
+
+import {a as x, set_a as set_x} from "modules-skip-1.js"