Fix a bug in since_sdk_version.dart

Discovered while attempting to migrate the file, split out for easier
review.

Change-Id: I1a3c56e0c277c79e9e71db89c8d5c8aa26891cc7
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/403283
Reviewed-by: Konstantin Shcheglov <[email protected]>
Commit-Queue: Brian Wilkerson <[email protected]>

Author: bwilkerson

pkg/analyzer/lib/src/dart/element/since_sdk_version.dart

@@ -60,7 +60,7 @@ class SinceSdkVersionComputer {
         }
       }
     }
-    return null;
+    return result;
   }
 
   /// Returns the parsed [Version], or `null` if wrong format.

pkg/analyzer/test/src/summary/elements/since_sdk_version_test.dart

@@ -384,6 +384,7 @@ library
         foo
           reference: dart:foo::@class::A::@method::foo
           firstFragment: dart:foo::<fragment>::@class::A::@method::foo
+          sinceSdkVersion: 2.16.0
 ''');
   }
 
@@ -440,6 +441,7 @@ library
         foo
           reference: dart:foo::@class::A::@method::foo
           firstFragment: dart:foo::<fragment>::@class::A::@method::foo
+          sinceSdkVersion: 2.14.0
 ''');
   }
 
@@ -906,6 +908,7 @@ library
     foo
       reference: dart:foo::@function::foo
       firstFragment: dart:foo::<fragment>::@function::foo
+      sinceSdkVersion: 2.15.0
       returnType: void
     bar
       reference: dart:foo::@function::bar
@@ -950,6 +953,7 @@ library
     foo
       reference: dart:foo::@function::foo
       firstFragment: dart:foo::<fragment>::@function::foo
+      sinceSdkVersion: 2.15.3-dev.7
       returnType: void
 ''');
   }
@@ -990,6 +994,7 @@ library
     foo
       reference: dart:foo::@function::foo
       firstFragment: dart:foo::<fragment>::@function::foo
+      sinceSdkVersion: 2.15.3
       returnType: void
 ''');
   }
@@ -1132,6 +1137,7 @@ library
         optionalNamed p2
           firstFragment: dart:foo::<fragment>::@function::f::@parameter::p2
           type: int?
+          sinceSdkVersion: 2.15.0
       returnType: void
 ''');
   }
@@ -1189,6 +1195,7 @@ library
           type: int
         optionalPositional p2
           type: int?
+          sinceSdkVersion: 2.15.0
       returnType: void
 ''');
   }
@@ -1227,6 +1234,7 @@ library
   typeAliases
     A
       firstFragment: dart:foo::<fragment>::@typeAlias::A
+      sinceSdkVersion: 2.15.0
       aliasedType: List<int>
 ''');
   }
@@ -1280,6 +1288,7 @@ library
     final hasInitializer foo
       reference: dart:foo::@topLevelVariable::foo
       firstFragment: dart:foo::<fragment>::@topLevelVariable::foo
+      sinceSdkVersion: 2.15.0
       type: int
       getter: dart:foo::<fragment>::@getter::foo#element
   getters