Fix file caching

One of the main advantage of the new build plugin API introduced in
Meteor 1.2 was the possibility to define "incremental compilers" that
relies on caching to accelerate the build process.

Unfortunately 7734099 didn't implement this new caching mechanism
correctly, this commit should make it work.

The documentation for CachingCompiler is available at:

  https://github.com/meteor/meteor/tree/devel/packages/caching-compiler

packages/jade/plugin/handler.js

@@ -15,11 +15,8 @@ class JadeCompilerPlugin extends CachingCompiler {
   }
 
   compileOneFile(file) {
-    const ext = file.getExtension();
-
     try {
-      const handlerMode = (ext === 'jade') ? 'file' : 'template';
-      this[`_${handlerMode}ModeHandler`](file);
+      return this._getCompilerResult(file);
     } catch (err) {
       file.error({
         message: "Jade syntax error: " + err.message
@@ -31,6 +28,24 @@ class JadeCompilerPlugin extends CachingCompiler {
     return [ inputFile.getSourceHash() ];
   }
 
+  addCompileResult(inputFile, compileResult) {
+    const fileMode = this._getMode(inputFile);
+    this[`_${fileMode}ModeHandler`](inputFile, compileResult);
+  }
+
+  compileResultSize(compileResult) {
+    function lengthOrZero(field) {
+      return field ? field.length : 0;
+    }
+    return lengthOrZero(compileResult.head) + lengthOrZero(compileResult.body) +
+      lengthOrZero(compileResult.templates);
+  }
+
+  _getMode(file) {
+    const ext = file.getExtension();
+    return (ext === 'jade') ? 'file' : 'template';
+  }
+
   // XXX Handle body attributes
   _bodyGen(tpl, attrs) {
     const renderFunction = SpacebarsCompiler.codeGen(tpl, {
@@ -60,11 +75,11 @@ class JadeCompilerPlugin extends CachingCompiler {
     `;
   }
 
-  _getCompilerResult(file, fileMode) {
+  _getCompilerResult(file) {
     try {
       return JadeCompiler.parse(file.getContentsAsString(), {
         filename: file.getPathInPackage(),
-        fileMode: fileMode
+        fileMode: this._getMode(file) === 'file'
       });
     } catch (err) {
       return file.error({
@@ -74,9 +89,7 @@ class JadeCompilerPlugin extends CachingCompiler {
     }
   }
 
-  _fileModeHandler(file) {
-    const results = this._getCompilerResult(file, true);
-
+  _fileModeHandler(file, results) {
     // Head
     if (results.head !== null) {
       file.addHtml({
@@ -102,8 +115,7 @@ class JadeCompilerPlugin extends CachingCompiler {
     }
   }
 
-  _templateModeHandler(file) {
-    const result = this._getCompilerResult(file, false);
+  _templateModeHandler(file, result) {
     const templateName = path.basename(file.getPathInPackage(), '.tpl.jade');
     let jsContent;