fix(core): fix placeholders handling in i18n.

Prior to this commit, translations were built in the serializers. This
could not work as a single translation can be used for different source
messages having different placeholder content.

Serializers do not try to replace the placeholders any more.
Placeholders are replaced by the translation bundle and the source
message is given as parameter so that the content of the placeholders is
taken into account.

Also XMB ids are now independent of the expression which is replaced by
a placeholder in the extracted file.
fixes #12512

modules/@angular/compiler/test/ml_parser/ast_serializer_spec.ts

@@ -97,4 +97,4 @@ const serializerVisitor = new _SerializerVisitor();
 
 export function serializeNodes(nodes: html.Node[]): string[] {
   return nodes.map(node => node.visit(serializerVisitor, null));
-}
+}