__non_webpack_require__ -> require in SystemJS

When running browser tests, e.g. via `gulp unittest`, the test files are not
processed by babel, and neither by the "unittestcli" gulp target.

This commit copies the babelPluginReplaceNonWebPackRequire plugin from the
unittestcli target to the SystemJS config so that `__non_webpack_require__` is
replaced with `require` for all build targets, and adds a unit test to ensure
that this indeed works as expected.

test/unit/display_svg_spec.js

@@ -108,6 +108,21 @@ describe('SVGGraphics', function () {
       });
     }
 
+    it('should fail require("zlib") unless in Node.js', function() {
+      function testFunc() {
+        __non_webpack_require__('zlib');
+      }
+      // Verifies that the script loader replaces __non_webpack_require__ with
+      // require.
+      expect(testFunc.toString()).toMatch(/\srequire\(["']zlib["']\)/);
+      if (isNodeJS()) {
+        expect(testFunc).not.toThrow();
+      } else {
+        // require not defined, require('zlib') not a module, etc.
+        expect(testFunc).toThrow();
+      }
+    });
+
     it('should produce a reasonably small svg:image', function(done) {
       if (!isNodeJS()) {
         pending('zlib.deflateSync is not supported in non-Node environments.');