Support "pub downgrade".

sdk/lib/_internal/pub/test/version_solver_test.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.
 
 library pub_upgrade_test;
 
 import 'dart:async';
 
 import 'package:unittest/unittest.dart';
 
@@ skipping 25 matching lines @@
   group('basic graph', basicGraph);
   group('with lockfile', withLockFile);
   group('root dependency', rootDependency);
   group('dev dependency', devDependency);
   group('unsolvable', unsolvable);
   group('bad source', badSource);
   group('backtracking', backtracking);
   group('SDK constraint', sdkConstraint);
   group('pre-release', prerelease);
   group('override', override);
+  group('downgrade', downgrade);
 }
 
 void basicGraph() {
   testResolve('no dependencies', {
     'myapp 0.0.0': {}
   }, result: {
     'myapp from root': '0.0.0'
   });
 
   testResolve('simple dependency tree', {
@@ skipping 994 matching lines @@
     },
     'foo 0.0.0': {}
   }, overrides: {
     'foo': 'any'
   }, result: {
     'myapp from root': '0.0.0',
     'foo': '0.0.0'
   });
 }
 
+void downgrade() {
+  testResolve("downgrades a dependency to the lowest matching version", {
+    'myapp 0.0.0': {
+      'foo': '>=2.0.0 <3.0.0'
+    },
+    'foo 1.0.0': {},
+    'foo 2.0.0-dev': {},
+    'foo 2.0.0': {},
+    'foo 2.1.0': {}
+  }, lockfile: {
+    'foo': '2.1.0'
+  }, result: {
+    'myapp from root': '0.0.0',
+    'foo': '2.0.0'
+  }, downgrade: true);
+
+  testResolve('use earliest allowed prerelease if no stable versions match '
+      'while downgrading', {
+    'myapp 0.0.0': {
+      'a': '>=2.0.0-dev.1 <3.0.0'
+    },
+    'a 1.0.0': {},
+    'a 2.0.0-dev.1': {},
+    'a 2.0.0-dev.2': {},
+    'a 2.0.0-dev.3': {}
+  }, result: {
+    'myapp from root': '0.0.0',
+    'a': '2.0.0-dev.1'
+  }, downgrade: true);
+}
+
 testResolve(String description, Map packages, {
     Map lockfile, Map overrides, Map result, FailMatcherBuilder error,
-    int maxTries}) {
+    int maxTries, bool downgrade: false}) {
   _testResolve(test, description, packages, lockfile: lockfile,
-      overrides: overrides, result: result, error: error, maxTries: maxTries);
+      overrides: overrides, result: result, error: error, maxTries: maxTries,
+      downgrade: downgrade);
 }
 
 solo_testResolve(String description, Map packages, {
     Map lockfile, Map overrides, Map result, FailMatcherBuilder error,
-    int maxTries}) {
+    int maxTries, bool downgrade: false}) {
   log.verbosity = log.Verbosity.SOLVER;
   _testResolve(solo_test, description, packages, lockfile: lockfile,
-      overrides: overrides, result: result, error: error, maxTries: maxTries);
+      overrides: overrides, result: result, error: error, maxTries: maxTries,
+      downgrade: downgrade);
 }
 
 _testResolve(void testFn(String description, Function body),
     String description, Map packages, {
     Map lockfile, Map overrides, Map result, FailMatcherBuilder error,
-    int maxTries}) {
+    int maxTries, bool downgrade: false}) {
   if (maxTries == null) maxTries = 1;
 
   testFn(description, () {
     var cache = new SystemCache('.');
     source1 = new MockSource('mock1');
     source2 = new MockSource('mock2');
     cache.register(source1);
     cache.register(source2);
     cache.sources.setDefault(source1.name);
 
@@ skipping 28 matching lines @@
     var realLockFile = new LockFile.empty();
     if (lockfile != null) {
       lockfile.forEach((name, version) {
         version = new Version.parse(version);
         realLockFile.packages[name] =
             new PackageId(name, source1.name, version, name);
       });
     }
 
     // Resolve the versions.
-    var future = resolveVersions(cache.sources, root, lockFile: realLockFile);
+    var future = resolveVersions(
+        downgrade ? SolveType.DOWNGRADE : SolveType.GET,
+        cache.sources, root, lockFile: realLockFile);
 
     var matcher;
     if (result != null) {
       matcher = new SolveSuccessMatcher(result, maxTries);
     } else if (error != null) {
       matcher = error(maxTries);
     }
 
     expect(future, completion(matcher));
   });
@@ skipping 182 matching lines @@
   /// that a source is only hit once for a given package and that pub
   /// internally caches the results.
   final _requestedVersions = new Set<String>();
 
   /// Keeps track of which package pubspecs have been requested. Ensures that a
   /// source is only hit once for a given package and that pub internally
   /// caches the results.
   final _requestedPubspecs = new Map<String, Set<Version>>();
 
   final String name;
+  final hasMultipleVersions = true;
 
   MockSource(this.name);
 
   dynamic parseDescription(String containingPath, description,
                              {bool fromLockFile: false}) => description;
 
   bool descriptionsEqual(description1, description2) =>
       description1 == description2;
 
   Future<String> getDirectory(PackageId id) {
@@ skipping 133 matching lines @@
   }
 
   var source = "mock1";
   if (match[7] != null) {
     source = match[7];
     if (source == "root") source = null;
   }
 
   return new PackageId(name, source, parsedVersion, description);
 }