chore(build): execute pub get only if a pubspec.yaml changed and run dart analyzer on all dart files

`pub get` is now only executed when the `pubspec.yaml` in the `modules`
folder is different than the `pubspec.yaml` in the `build/dart` folder.

Generates the file `build/dart/_analyzer.dart` that imports all modules
to run `dart analyzer` against all of them. The build will fail whenever
there are errors, warnings or hints in `dart analyzer`.

Changes the sources so that `dart analyzer` does not report any
error, warning or hint.

Closes #40

modules/facade/pubspec.yaml

@@ -5,3 +5,4 @@ dependencies:
 dev_dependencies:
   test_lib:
     path: ../test_lib
+  guinness: ">=0.1.5 <0.2.0"

modules/facade/src/dom.dart

@@ -6,7 +6,7 @@ export 'dart:html' show DocumentFragment, Node, Element, TemplateElement, Text;
 
 class DOM {
   static query(selector) {
-    return document.query(selector);
+    return document.querySelector(selector);
   }
   static on(element, event, callback) {
     element.addEventListener(event, callback);

modules/facade/src/lang.dart

@@ -4,12 +4,20 @@ export 'dart:async' show Future;
 export 'dart:core' show Type, int;
 
 class FIELD {
-  const constructor(this.definition);
+  final String definition;
+  const FIELD(this.definition);
 }
 
-class CONST {}
-class ABSTRACT {}
-class IMPLEMENTS {}
+class CONST {
+  const CONST();
+}
+class ABSTRACT {
+  const ABSTRACT();
+}
+class IMPLEMENTS {
+  final interfaceClass;
+  const IMPLEMENTS(this.interfaceClass);
+}
 
 
 class StringWrapper {