Add validation for Flutter constraints.

test/validator/sdk_constraint_test.dart

Index: test/validator/sdk_constraint_test.dart
diff --git a/test/validator/sdk_constraint_test.dart b/test/validator/sdk_constraint_test.dart
index ce09d7110133295432f023d7964a619b55a098cc..ace760bc6777731545bd363bdf00943d625968da 100644
--- a/test/validator/sdk_constraint_test.dart
+++ b/test/validator/sdk_constraint_test.dart
@@ -27,14 +27,68 @@ main() {
       ]).create();
       expectNoValidationError(sdkConstraint);
     });
+
+    integration('has a Flutter SDK constraint with an appropriate Dart SDK '
+        'constraint', () {
+      d.dir(appPath, [
+        d.pubspec({
+          "name": "test_pkg",
+          "version": "1.0.0",
+          "environment": {
+            "sdk": ">=1.19.0 <2.0.0",
+            "flutter": "^1.2.3"
+          }
+        })
+      ]).create();
+      expectNoValidationError(sdkConstraint);
+    });
   });
 
-  test("should consider a package invalid if it has an SDK constraint with "
-      "^", () {
-    d.dir(appPath, [
-      d.libPubspec("test_pkg", "1.0.0", sdk: "^1.8.0")
-    ]).create();
-    expect(schedulePackageValidation(sdkConstraint),
-        completion(pairOf(anyElement(contains('">=1.8.0 <2.0.0"')), isEmpty)));
+  group("should consider a package invalid if it", () {
+    integration("has an SDK constraint with ^", () {
+      d.dir(appPath, [
+        d.libPubspec("test_pkg", "1.0.0", sdk: "^1.8.0")
+      ]).create();
+      expect(
+          schedulePackageValidation(sdkConstraint),
+          completion(pairOf(
+              anyElement(contains('">=1.8.0 <2.0.0"')),
+              isEmpty)));
+    });
+
+    integration("has a Flutter SDK constraint with a too-broad SDK "
+        "constraint", () {
+      d.dir(appPath, [
+        d.pubspec({
+          "name": "test_pkg",
+          "version": "1.0.0",
+          "environment": {
+            "sdk": ">=1.18.0 <1.50.0",
+            "flutter": "^1.2.3"
+          }
+        })
+      ]).create();
+      expect(
+          schedulePackageValidation(sdkConstraint),
+          completion(pairOf(
+              anyElement(contains('">=1.19.0 <1.50.0"')),
+              isEmpty)));
+    });
+
+    integration("has a Flutter SDK constraint with no SDK "
+        "constraint", () {
+      d.dir(appPath, [
+        d.pubspec({
+          "name": "test_pkg",
+          "version": "1.0.0",
+          "environment": {"flutter": "^1.2.3"}
+        })
+      ]).create();
+      expect(
+          schedulePackageValidation(sdkConstraint),
+          completion(pairOf(
+              anyElement(contains('">=1.19.0 <2.0.0"')),
+              isEmpty)));
+    });
   });
 }