Avoid source-map import under AMD

Fixes #989

lib/handlebars/compiler/code-gen.js

@@ -1,9 +1,19 @@
 import {isArray} from "../utils";
 
+var SourceNode;
+
 try {
-  var SourceMap = require('source-map'),
-        SourceNode = SourceMap.SourceNode;
+  if (typeof define !== 'function' || !define.amd) {
+    // We don't support this in AMD environments. For these environments, we asusme that
+    // they are running on the browser and thus have no need for the source-map library.
+    var SourceMap = require('source-map');
+    SourceNode = SourceMap.SourceNode;
+  }
 } catch (err) {
+  /* NOP */
+}
+
+if (!SourceNode) {
   /* istanbul ignore next: tested but not covered in istanbul due to dist build  */
   SourceNode = function(line, column, srcFile, chunks) {
     this.src = '';

spec/source-map.js

@@ -1,7 +1,9 @@
 /*global CompilerContext, Handlebars */
 try {
-  var SourceMap = require('source-map'),
-        SourceMapConsumer = SourceMap.SourceMapConsumer;
+  if (typeof define !== 'function' || !define.amd) {
+    var SourceMap = require('source-map'),
+          SourceMapConsumer = SourceMap.SourceMapConsumer;
+  }
 } catch (err) {
   /* NOP for in browser */
 }