chore(docgen): Use updated dartdoc tool

Use the new `dartdoc` tool, which
[shipped with 1.12](https://groups.google.com/a/dartlang.org/forum/#!topic/announce/Q1JSlVaFFnY)

Update the sdk constraint to 1.12

tools/build/dart.js

@@ -8,7 +8,7 @@ module.exports.detect = function(gulp) {
     if (process.platform === 'win32') {
       DART_SDK = {
         ANALYZER: 'dartanalyzer.bat',
-        DARTDOCGEN: 'dartdocgen.bat',
+        DARTDOCGEN: 'dartdoc.bat',
         DARTFMT: 'dartfmt.bat',
         PUB: 'pub.bat',
         VM: 'dart.exe'
@@ -16,7 +16,7 @@ module.exports.detect = function(gulp) {
     } else {
       DART_SDK = {
         ANALYZER: 'dartanalyzer',
-        DARTDOCGEN: 'dartdocgen',
+        DARTDOCGEN: 'dartdoc',
         DARTFMT: 'dartfmt',
         PUB: 'pub',
         VM: 'dart'