Improve circular dependency overrides.

When a dependency is overridden, any dependency it has back on the root
package is now ignored. This makes it easier for users to develop one
portion of a circular dependency.

Closes #1443

R=rnystrom@google.com

Review URL: https://codereview.chromium.org//2276943003 .

lib/src/solver/backtracking_solver.dart

@@ -579,6 +579,13 @@ class BacktrackingSolver {
     } else {
       // Ignore any overridden dependencies.
       deps.removeWhere((dep) => _overrides.containsKey(dep.name));
+
+      // If an overridden dependency depends on the root package, ignore that
+      // dependency. This ensures that users can work on the next version of one
+      // side of a circular dependency easily.
+      if (_overrides.containsKey(id.name)) {
+        deps.removeWhere((dep) => dep.name == root.name);
+      }
     }
 
     // Make sure the package doesn't have any bad dependencies.

test/version_solver_test.dart

@@ -1216,7 +1216,7 @@ void override() {
                '- myapp depends on version >=1.0.0 <2.0.0');
   });
 
-  integration('override a bad source without error', () {
+  integration('overrides a bad source without error', () {
     servePackages((builder) {
       builder.serve('foo', '0.0.0');
     });
@@ -1231,6 +1231,22 @@ void override() {
 
     expectResolves(result: {'foo': '0.0.0'});
   });
+
+  integration('overrides an unmatched root dependency', () {
+    servePackages((builder) {
+      builder.serve('foo', '0.0.0', deps: {'myapp': '1.0.0'});
+    });
+
+    d.dir(appPath, [
+      d.pubspec({
+        'name': 'myapp',
+        'version': '2.0.0',
+        'dependency_overrides': {'foo': 'any'}
+      })
+    ]).create();
+
+    expectResolves(result: {'foo': '0.0.0'});
+  });
 }
 
 void downgrade() {