Don't shake variables as computed property in member expressions (#3685)

* Add test

* Fix shaking of computed property accesses

* More exhaustive test

packages/core/integration-tests/test/integration/scope-hoisting/es6/tree-shaking-export-computed-prop/a.js

@@ -0,0 +1,4 @@
+import b from './b.js';
+import c from './b.js';
+
+output = [b('abc'), b('x'), c('abc'), c('x')];

packages/core/integration-tests/test/integration/scope-hoisting/es6/tree-shaking-export-computed-prop/b.js

@@ -0,0 +1,8 @@
+let x = "abc",
+  y = "def";
+let data = {};
+data[x] = data[y] = true;
+
+export default function(a) {
+  return data[a];
+}

packages/core/integration-tests/test/integration/scope-hoisting/es6/tree-shaking-export-computed-prop/c.js

@@ -0,0 +1,9 @@
+let x = "abc",
+  y = "def";
+let data = {};
+data[x] = true;
+data[y] = true;
+
+export default function(a) {
+	return data[a];
+}

packages/core/integration-tests/test/scope-hoisting.js

@@ -570,6 +570,22 @@ describe('scope hoisting', function() {
       assert(!contents.includes('method'));
     });
 
+    it('keeps member expression with computed properties that are variables', async function() {
+      let b = await bundle(
+        path.join(
+          __dirname,
+          '/integration/scope-hoisting/es6/tree-shaking-export-computed-prop/a.js'
+        ),
+        {minify: true}
+      );
+
+      let output = await run(b);
+      assert.strictEqual(output[0], true);
+      assert.strictEqual(typeof output[1], 'undefined');
+      assert.strictEqual(output[2], true);
+      assert.strictEqual(typeof output[3], 'undefined');
+    });
+
     it('support exporting a ES6 module exported as CommonJS', async function() {
       let b = await bundle(
         path.join(

packages/shared/scope-hoisting/src/shake.js

@@ -76,8 +76,9 @@ function isPure(binding) {
 
 function isExportAssignment(path) {
   return (
-    // match "path.any = any;"
+    // match "path.foo = bar;"
     path.parentPath.isMemberExpression() &&
+    path.parentPath.node.object === path.node &&
     path.parentPath.parentPath.isAssignmentExpression() &&
     path.parentPath.parentPath.node.left === path.parentPath.node
   );