Don't adjust SDK constraints whose min versions are prerelease

Previously, if the user was using 1.2.3-dev.1 and a package had an SDK
constraint ">=1.2.3-dev.2", we would try to create an invalid
">=1.2.3-dev.2 <1.2.3-dev.1" constraint.

Closes #1770

Author: nex3

lib/src/pubspec.dart

@@ -391,6 +391,9 @@ class Pubspec {
     if (!allowPreReleaseSdk) return false;
     if (!sdk.version.isPreRelease) return false;
     if (sdkConstraint.includeMax) return false;
+    if (sdkConstraint.min != null && sdkConstraint.min.isPreRelease) {
+      return false;
+    }
     if (sdkConstraint.max == null) return false;
     if (sdkConstraint.max.isPreRelease) return false;
     return sdkConstraint.max.major == sdk.version.major &&

test/version_solver_test.dart

@@ -1203,7 +1203,7 @@ void dartSdkConstraint() {
                 'SDK is 1.2.3.');
       });
 
-      test("no existing pre-release constraint", () async {
+      test("no max pre-release constraint", () async {
         await d.dir(appPath, [
           await d.pubspec({
             'name': 'myapp',
@@ -1216,6 +1216,20 @@ void dartSdkConstraint() {
             output: isNot(contains('PUB_ALLOW_PRERELEASE_SDK')));
       });
 
+      test("no min pre-release constraint", () async {
+        await d.dir(appPath, [
+          await d.pubspec({
+            'name': 'myapp',
+            'environment': {'sdk': '>=1.2.3-dev.2.0 <2.0.0'}
+          })
+        ]).create();
+
+        await expectResolves(
+            environment: {'_PUB_TEST_SDK_VERSION': '1.2.3-dev.1.0'},
+            error: 'Package myapp requires SDK version >=1.2.3-dev.2.0 <2.0.0 '
+                'but the current SDK is 1.2.3-dev.1.0.');
+      });
+
       test("no build release constraints", () async {
         await d.dir(appPath, [
           await d.pubspec({