[Resolver] Ensure Informative errors are always shown for platform incompatibilities

Closes https://github.com/CocoaPods/CocoaPods/issues/2868

lib/cocoapods/resolver.rb

@@ -50,7 +50,7 @@ module Pod
     #         definition.
     #
     def resolve
-      dependencies = @podfile.target_definition_list.map(&:dependencies).flatten
+      dependencies = podfile.target_definition_list.map(&:dependencies).flatten
       @cached_sets = {}
       @activated = Molinillo::Resolver.new(self, self).resolve(dependencies, locked_dependencies)
       specs_by_target
@@ -78,7 +78,6 @@ module Pod
             uniq.
             sort_by(&:name).
             each do |spec|
-              validate_platform(spec, target)
               sandbox.store_head_pod(spec.name) if spec.version.head?
             end
         end
@@ -408,6 +407,7 @@ module Pod
     #         dependencies for `target`.
     #
     def valid_dependencies_for_target_from_node(target, node)
+      validate_platform(node.payload, target)
       dependency_nodes = node.outgoing_edges.select do |edge|
         edge_is_valid_for_target?(edge, target)
       end.map(&:destination)

spec/unit/resolver_spec.rb

@@ -175,6 +175,17 @@ module Pod
         e.message.should.match(/platform .* not compatible/)
       end
 
+      it 'raises when a resolved dependency has a platform incompatibility' do
+        @podfile = Podfile.new do
+          platform :osx, '10.7'
+          pod 'ReactiveCocoa', '0.16.1' # this version is iOS-only
+        end
+        @resolver.stubs(:podfile).returns(@podfile)
+        should.raise Informative do
+          @resolver.resolve
+        end.message.should.match /platform .* not compatible/
+      end
+
       it 'raises if unable to find a specification' do
         Specification.any_instance.stubs(:all_dependencies).returns([Dependency.new('Windows')])
         message = should.raise Informative do