Add a method to check for strong-named assemblies

Add a utility method to check for strong-named assemblies.

`AssemblyUtilities.HasStrongName` uses the metadata reader to check if
the assembly at a given path has a strong name or not.

This commit includes the code itself and a couple of unit tests. A later
commit will wire it into the compilers/VS.

src/Compilers/Core/CodeAnalysisTest/AssemblyUtilitiesTests.cs

@@ -213,5 +213,23 @@ public void IdentifyMissingDependencies_MultipleMissing()
             Assert.Contains("mscorlib", results);
             Assert.Contains("Gamma", results);
         }
+
+        [Fact]
+        public void HasStrongName_False()
+        {
+            var directory = Temp.CreateDirectory();
+
+            var alphaDll = directory.CreateFile("Alpha.dll").WriteAllBytes(TestResources.AssemblyLoadTests.AssemblyLoadTests.Alpha);
+
+            Assert.False(AssemblyUtilities.HasStrongName(alphaDll.Path));
+        }
+
+        [Fact]
+        public void HasStrongName_True()
+        {
+            string path = Assembly.GetExecutingAssembly().Location;
+
+            Assert.True(AssemblyUtilities.HasStrongName(path));
+        }
     }
 }

src/Compilers/Core/Portable/AssemblyUtilities.cs

@@ -130,6 +130,10 @@ public static ImmutableArray<string> FindSatelliteAssemblies(string filePath)
             return builder.ToImmutable();
         }
 
+        /// <summary>
+        /// Given a path to an assembly and a set of paths to possible dependencies,
+        /// identifies which of the assembly's references are missing.
+        /// </summary>
         public static ImmutableArray<AssemblyIdentity> IdentifyMissingDependencies(string assemblyPath, IEnumerable<string> assemblySet)
         {
             if (assemblyPath == null)
@@ -168,5 +172,25 @@ public static ImmutableArray<AssemblyIdentity> IdentifyMissingDependencies(strin
 
             return ImmutableArray.CreateRange(assemblyReferences);
         }
+
+        /// <summary>
+        /// Given a path to an assembly, returns true if the assembly has a strong
+        /// name (either a full key or a token) or false otherwise.
+        /// </summary>
+        public static bool HasStrongName(string assemblyPath)
+        {
+            if (assemblyPath == null)
+            {
+                throw new ArgumentNullException(nameof(assemblyPath));
+            }
+
+            using (var reader = new PEReader(FileUtilities.OpenRead(assemblyPath)))
+            {
+                var metadataReader = reader.GetMetadataReader();
+                var assemblyDefinition = metadataReader.ReadAssemblyIdentityOrThrow();
+
+                return assemblyDefinition.IsStrongName;
+            }
+        }
     }
 }