Support "pub downgrade".

sdk/lib/_internal/pub/test/version_solver_test.dart

Index: sdk/lib/_internal/pub/test/version_solver_test.dart
diff --git a/sdk/lib/_internal/pub/test/version_solver_test.dart b/sdk/lib/_internal/pub/test/version_solver_test.dart
index b5302cb7e498c7d0b9642f53c65b9274ee5450e8..817d88819ead6db1a0249fb30eee5584f7ff676b 100644
--- a/sdk/lib/_internal/pub/test/version_solver_test.dart
+++ b/sdk/lib/_internal/pub/test/version_solver_test.dart
@@ -924,6 +924,20 @@ void prerelease() {
     'myapp from root': '0.0.0',
     'a': '1.1.0'
   });

[Bob Nystrom, 2014/07/09 18:19:00]

It would be nice to have unit tests for the other downgrade logic here too. I
know it's more or less redundant with the integration tests, but it's nice being
able to quickly test the solver code in isolation.

[nweiz, 2014/07/09 21:28:58]

I thought this code was mostly for testing behavior that was common to all types
of solving. There aren't any tests for upgrade in particular here, for example.

I can copy some tests here if you want, but it seems like a bunch of extra work
for no actual coverage benefit.

[Bob Nystrom, 2014/07/10 17:46:11]

It tries to test all of the solving behavior that isn't source specific.
The withLockfile() tests, I think, cover a lot of that.
Instead of copying, I'd be fine with removing the integration tests for
downgrades_dependency, unlock_dependers, and unlock_if_necessary and only
testing them here. Those tests will run much faster and be shorter in here, I
think.

[nweiz, 2014/07/14 21:52:05]

I moved over downgrades_dependency, but the other two need to be run on a
specific package so can't really be ported without adding a bunch of
functionality to the version solver tests.

+
+  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);
 }
 
 void override() {
@@ -1059,23 +1073,25 @@ void override() {
 
 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, () {
@@ -1124,7 +1140,9 @@ _testResolve(void testFn(String description, Function body),
     }
 
     // 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) {