Report a deprecation warning when 'use 6to5' is used instead of 'use babel'.

src/babel.coffee

@@ -8,6 +8,7 @@ crypto = require 'crypto'
 fs = require 'fs-plus'
 path = require 'path'
 babel = null # Defer until used
+Grim = null # Defer until used
 
 stats =
   hits: 0
@@ -127,15 +128,33 @@ transpile = (sourceCode, filePath, cachePath) ->
 
   js
 
+isRoot = (filePath) ->
+  parts = path.parse(filePath)
+  parts.root == filePath
+
 # Function that obeys the contract of an entry in the require.extensions map.
 # Returns the transpiled version of the JavaScript code at filePath, which is
 # either generated on the fly or pulled from cache.
 loadFile = (module, filePath) ->
   sourceCode = fs.readFileSync(filePath, 'utf8')
-  return module._compile(sourceCode, filePath) unless sourceCode.startsWith('"use 6to5"') or
-    sourceCode.startsWith("'use 6to5'") or
-    sourceCode.startsWith('"use babel"') or
-    sourceCode.startsWith("'use babel'")
+  if sourceCode.startsWith('"use babel"') or sourceCode.startsWith("'use babel'")
+    # Continue.
+  else if sourceCode.startsWith('"use 6to5"') or sourceCode.startsWith("'use 6to5'")
+    packageName
+    directory = filePath
+    until packageName
+      directory = path.dirname(directory)
+      manifest = path.join(directory, 'package.json')
+      if fs.existsSync(manifest)
+        json = JSON.parse(fs.readFileSync(manifest))
+        packageName = json.name
+      else if isRoot(directory)
+        break
+
+    Grim ?= require 'grim'
+    Grim.deprecate("Use the 'use babel' pragma instead of 'use 6to5' in #{filePath}", {packageName})
+  else
+    return module._compile(sourceCode, filePath)
 
   cachePath = getCachePath(sourceCode)
   js = getCachedJavaScript(cachePath) ? transpile(sourceCode, filePath, cachePath)