Generate source maps

Allow the precompiler to generate source maps when the srcFile parameter is passed.

This refactors large chunks of the code generation pipeline, allowing metadata to be associated with code chunks as well as breaking out much of the code generation logic into a separate helper.

spec/source-map.js

@@ -0,0 +1,45 @@
+/*global CompilerContext, Handlebars */
+var SourceMap = require('source-map'),
+      SourceMapConsumer = SourceMap.SourceMapConsumer;
+
+describe('source-map', function() {
+  if (!Handlebars.precompile) {
+    return;
+  }
+
+  it('should safely include source map info', function() {
+    var template = Handlebars.precompile('{{hello}}', {destName: 'dest.js', srcName: 'src.hbs'});
+
+    equal(!!template.code, true);
+    equal(!!template.map, !CompilerContext.browser);
+  });
+  it('should map source properly', function() {
+    var source = '  b{{hello}}  \n  {{bar}}a {{#block arg hash=(subex 1 subval)}}{{/block}}',
+        template = Handlebars.precompile(source, {destName: 'dest.js', srcName: 'src.hbs'});
+
+    if (template.map) {
+      var consumer = new SourceMapConsumer(template.map),
+          lines = template.code.split('\n'),
+          srcLines = source.split('\n'),
+
+          generated = grepLine('"  b"', lines),
+          source = grepLine('  b', srcLines);
+
+      var mapped = consumer.originalPositionFor(generated);
+      equal(mapped.line, source.line);
+      equal(mapped.column, source.column);
+    }
+  });
+});
+
+function grepLine(token, lines) {
+  for (var i = 0; i < lines.length; i++) {
+    var column = lines[i].indexOf(token);
+    if (column >= 0) {
+      return {
+        line: i+1,
+        column: column
+      };
+    }
+  }
+}