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/change_detection/src/parser/scanner.js

@@ -368,7 +368,7 @@ export class Scanner {
   }
 
   error(message:string) {
-    var position:int = this.index + this.offset;
+    var position:int = this.index;
     throw `Lexer Error: ${message} at column ${position} in expression [${input}]`;
   }
 }