Ensure a consistent return value for sourceContentFor

The sourceContentFor function was previously returning null when it
should have returned an exception when the source content was not
provided.

Author: andrewnicols

lib/source-map-consumer.js

@@ -522,12 +522,15 @@ class BasicSourceMapConsumer extends SourceMapConsumer {
 
   /**
    * Returns the original source content. The only argument is the url of the
-   * original source file. Returns null if no original source content is
-   * available.
+   * original source file.
    */
   sourceContentFor(aSource, nullOnMissing) {
     if (!this.sourcesContent) {
-      return null;
+      if (nullOnMissing) {
+        return null;
+      }
+
+      throw new Error('"' + aSource + '" is not in the SourceMap.');
     }
 
     const index = this._findSourceIndex(aSource);
@@ -822,8 +825,7 @@ class IndexedSourceMapConsumer extends SourceMapConsumer {
 
   /**
    * Returns the original source content. The only argument is the url of the
-   * original source file. Returns null if no original source content is
-   * available.
+   * original source file.
    */
   sourceContentFor(aSource, nullOnMissing) {
     for (let i = 0; i < this._sections.length; i++) {

lib/source-map-generator.js

@@ -79,7 +79,7 @@ class SourceMapGenerator {
         generator._sources.add(sourceRelative);
       }
 
-      const content = aSourceMapConsumer.sourceContentFor(sourceFile);
+      const content = aSourceMapConsumer.sourceContentFor(sourceFile, true);
       if (content != null) {
         generator.setSourceContent(sourceFile, content);
       }
@@ -238,7 +238,7 @@ class SourceMapGenerator {
 
     // Copy sourcesContents of applied map.
     aSourceMapConsumer.sources.forEach(function(srcFile) {
-      const content = aSourceMapConsumer.sourceContentFor(srcFile);
+      const content = aSourceMapConsumer.sourceContentFor(srcFile, true);
       if (content != null) {
         if (aSourceMapPath != null) {
           srcFile = util.join(aSourceMapPath, srcFile);

lib/source-node.js

@@ -137,7 +137,7 @@ class SourceNode {
 
     // Copy sourcesContent into SourceNode
     aSourceMapConsumer.sources.forEach(function(sourceFile) {
-      const content = aSourceMapConsumer.sourceContentFor(sourceFile);
+      const content = aSourceMapConsumer.sourceContentFor(sourceFile, true);
       if (content != null) {
         if (aRelativePath != null) {
           sourceFile = util.join(aRelativePath, sourceFile);