Dependency overrides override SDK constraints.

lib/src/solver/backtracking_solver.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /// A back-tracking depth-first solver.
 ///
 /// Attempts to find the best solution for a root package's transitive
 /// dependency graph, where a "solution" is a set of concrete package versions.
 /// A valid solution will select concrete versions for every package reached
 /// from the root package's dependency graph, and each of those packages will
@@ skipping 624 matching lines @@
         message = "* select ${_versions.last.current}";
       }
     } else {
       // Otherwise, indent it under the current selected package.
       message = prefixLines(message);
     }
 
     // Indent for the previous selections.
     log.solver(prefixLines(message, prefix: '| ' * _versions.length));
   }
+
+  /// Ensures that if [pubspec] has an SDK constraint, then it is compatible
+  /// with the current SDK.
+  ///
+  /// Throws a [SolveFailure] if not.
+  void _validateSdkConstraint(Pubspec pubspec) {
+    if (_overrides.containsKey(pubspec.name)) return;
+    if (pubspec.environment.sdkVersion.allows(sdk.version)) return;
+
+    throw new BadSdkVersionException(pubspec.name,
+        'Package ${pubspec.name} requires SDK version '
+        '${pubspec.environment.sdkVersion} but the current SDK is '
+        '${sdk.version}.');
+  }
 }
-
-/// Ensures that if [pubspec] has an SDK constraint, then it is compatible
-/// with the current SDK.
-///
-/// Throws a [SolveFailure] if not.
-void _validateSdkConstraint(Pubspec pubspec) {
-  if (pubspec.environment.sdkVersion.allows(sdk.version)) return;
-
-  throw new BadSdkVersionException(pubspec.name,
-      'Package ${pubspec.name} requires SDK version '
-      '${pubspec.environment.sdkVersion} but the current SDK is '
-      '${sdk.version}.');
-}