Prevent redefinition of class libraries

A number of classes used in PolyGerrit are defined in vanilla JS files
that are included multiple times by various elements.  For example,
gr-diff-line.js is included by the gr-diff-builder and by gr-diff
elements. Adds #ifndef-style guards to each of these libraries to
prevent redefinition and avoid different elements potentially referring
to different versions of the same class.

Change-Id: I45e3ba425a59989b328475b1fe58fd9f350c8ae0

polygerrit-ui/app/elements/diff/gr-diff-builder/gr-diff-builder-image.js

@@ -14,6 +14,9 @@
 (function(window, GrDiffBuilderSideBySide) {
   'use strict';
 
+  // Prevent redefinition.
+  if (window.GrDiffBuilderImage) { return; }
+
   function GrDiffBuilderImage(diff, comments, prefs, outputEl, baseImage,
       revisionImage) {
     GrDiffBuilderSideBySide.call(this, diff, comments, prefs, outputEl);

polygerrit-ui/app/elements/diff/gr-diff-builder/gr-diff-builder-side-by-side.js

@@ -14,6 +14,9 @@
 (function(window, GrDiffBuilder) {
   'use strict';
 
+  // Prevent redefinition.
+  if (window.GrDiffBuilderSideBySide) { return; }
+
   function GrDiffBuilderSideBySide(diff, comments, prefs, outputEl) {
     GrDiffBuilder.call(this, diff, comments, prefs, outputEl);
   }

polygerrit-ui/app/elements/diff/gr-diff-builder/gr-diff-builder-unified.js

@@ -14,6 +14,9 @@
 (function(window, GrDiffBuilder) {
   'use strict';
 
+  // Prevent redefinition.
+  if (window.GrDiffBuilderUnified) { return; }
+
   function GrDiffBuilderUnified(diff, comments, prefs, outputEl) {
     GrDiffBuilder.call(this, diff, comments, prefs, outputEl);
   }

polygerrit-ui/app/elements/diff/gr-diff-builder/gr-diff-builder.js

@@ -14,6 +14,9 @@
 (function(window, GrDiffGroup, GrDiffLine) {
   'use strict';
 
+  // Prevent redefinition.
+  if (window.GrDiffBuilder) { return; }
+
   function GrDiffBuilder(diff, comments, prefs, outputEl) {
     this._diff = diff;
     this._comments = comments;

polygerrit-ui/app/elements/diff/gr-diff/gr-diff-group.js

@@ -14,6 +14,9 @@
 (function(window, GrDiffLine) {
   'use strict';
 
+  // Prevent redefinition.
+  if (window.GrDiffGroup) { return; }
+
   function GrDiffGroup(type, opt_lines) {
     this.type = type;
     this.lines = [];

polygerrit-ui/app/elements/diff/gr-diff/gr-diff-line.js

@@ -14,6 +14,9 @@
 (function(window) {
   'use strict';
 
+  // Prevent redefinition.
+  if (window.GrDiffLine) { return; }
+
   function GrDiffLine(type) {
     this.type = type;
     this.highlights = [];