Pass async flag to jit in aot tools

src/coreclr/tools/Common/JitInterface/CorInfoImpl.cs

@@ -3713,7 +3713,7 @@ private bool getTailCallHelpers(ref CORINFO_RESOLVED_TOKEN callToken, CORINFO_SI
         private CORINFO_METHOD_STRUCT_* getAsyncResumptionStub()
 #pragma warning restore CA1822 // Mark members as static
         {
-            return null;
+            throw new NotImplementedException("Crossgen2 does not support runtime-async yet");
         }
 
         private byte[] _code;
@@ -4297,6 +4297,11 @@ private uint getJitFlags(ref CORJIT_FLAGS flags, uint sizeInBytes)
                 flags.Set(CorJitFlag.CORJIT_FLAG_SOFTFP_ABI);
             }
 
+            if (this.MethodBeingCompiled.IsRuntimeAsync)
+            {
+                flags.Set(CorJitFlag.CORJIT_FLAG_ASYNC);
+            }
+
             return (uint)sizeof(CORJIT_FLAGS);
         }
 

src/coreclr/tools/Common/JitInterface/CorInfoTypes.cs

@@ -1414,6 +1414,7 @@ public enum CorJitFlag : uint
         // ARM only
         CORJIT_FLAG_RELATIVE_CODE_RELOCS    = 29, // JIT should generate PC-relative address computations instead of EE relocation records
         CORJIT_FLAG_SOFTFP_ABI              = 30, // Enable armel calling convention
+        CORJIT_FLAG_ASYNC                   = 31,  // Generate code for use as an async function
     }
 
     public struct CORJIT_FLAGS

src/coreclr/tools/Common/TypeSystem/Common/MethodDesc.cs

@@ -658,6 +658,14 @@ public virtual bool IsPublic
             }
         }
 
+        public virtual bool IsRuntimeAsync
+        {
+            get
+            {
+                return false;
+            }
+        }
+
         public abstract bool HasCustomAttribute(string attributeNamespace, string attributeName);
 
         /// <summary>

src/coreclr/tools/Common/TypeSystem/Ecma/EcmaMethod.cs

@@ -32,6 +32,7 @@ private static class MethodFlags
             public const int AttributeMetadataCache = 0x02000;
             public const int Intrinsic              = 0x04000;
             public const int UnmanagedCallersOnly   = 0x08000;
+            public const int Async                  = 0x10000;
         };
 
         private EcmaType _type;
@@ -167,6 +168,9 @@ private int InitializeMethodFlags(int mask)
                 if ((methodImplAttributes & MethodImplAttributes.Synchronized) != 0)
                     flags |= MethodFlags.Synchronized;
 
+                if ((methodImplAttributes & MethodImplAttributes.Async) != 0)
+                    flags |= MethodFlags.Async;
+
                 flags |= MethodFlags.BasicMetadataCache;
             }
 
@@ -367,6 +371,14 @@ public override bool IsStaticConstructor
             }
         }
 
+        public override bool IsRuntimeAsync
+        {
+            get
+            {
+                return (GetMethodFlags(MethodFlags.BasicMetadataCache | MethodFlags.Async) & MethodFlags.Async) != 0;
+            }
+        }
+
         public MethodAttributes Attributes
         {
             get